Author’s Note: In November, Blizzard Entertainment finally revealed Diablo IV to fans around the world. Long-time players rejoiced at all the similarities to 2000’s Diablo II: skill trees, familiar classes, and a gritty (yet still colorful and evocative) art style. There’s a reason why Blizzard is following the design of Diablo II — originally developed by the now-defunct Blizzard North — so closely. In many ways, Blizzard North’s beloved sequel is still the standard bearer for action-RPGs, also affectionately known as Diablo clones. My newest book, Stay Awhile and Listen: Book II — Heaven, Hell, and Secret Cow Levels, explores the making of Diablo II in-depth, and highlights what made its core game loop so long-lasting. The following excerpt from the book — available for pre-order ahead of its December 10 publication date — examines some of the moving parts of that game loop, and how they work together to keep players clicking for over 19 years and counting.
MANY PLAYERS, CRITICS, and game historians point to Diablo’s loot system as its core appeal. Games that procedurally generate loot are commonly referred to as Diablo-likes, even games outside the fantasy-RPG genre such as Gearbox’s Borderlands shooters.
Giving loot all the credit for the Diablo franchise’s success fails to acknowledge Blizzard North’s holistic vision. Diablo II’s game systems — gems, procedurally generated areas and loot, tiers of items, the feedback of mouse clicks, the sound cues of items popping out of monsters and treasure chests — weave together into a spider web-like design. Every strand of the web depends on the support of the others. Touch one, and the web may unravel.
The game loop begins with a mouse click. The avatar responds instantly, teaching players that the game will leap to obey their every click. The feedback of clicking on an enemy comes next. Speakers emit a solid thwack, slice, or stab depending on the type of weapon being used and whether the enemy is wearing armor. The animation of the swing, thrust, spell-casting, or bowstring is also pleasant: fast, yet not so fast that the animation looks shoddy.
Players keep clicking — or click and hold — a mouse button until their target dies. The target’s death animation should be strong: A Fallen flipping through the air and collapsing in a pool of blood; a Skeleton shattering into a heap of splintered bones; a Zombie crumbling into dismembered limbs. That animation is tantamount to pulling the lever on a slot machine. The sight and sound of an item bursting free of a defeated monster means the machine has decided in their favor — but to what degree? And what effect should the sound of that item appearing in the world have on the player’s psyche?
DIABLO II BALLOONED to approximately four times the size of its predecessor. Even as the project swelled, Blizzard North’s developers worked on a micro scale, polishing individual elements until they sparkled.
“Max always had this saying: ‘There’s never enough blood and fire.’ That was what he pushed the artists to do: more fire, more blood,” remembered Michael Huang, IT engineer.
Max Schaefer’s emphasis on blood and fire was less suggestion and more ideology. Taken literally, it amounted to window dressing. Andariel’s lair at the end of Act One is painted in flames and pits of bubbling blood. Figuratively, “blood and fire” referred to small touches that made big impacts on the game, the action-RPG genre, Blizzard North, and the gaming industry.
A smooth difficulty curve was one such blood-and-fire change. “One big challenge, which I felt resulted in a big win, was establishing a formulaic difficulty curve for the game,” explained Stieg Hedlund, designer. When brainstorming enemies with character artists, Stieg thought about how enemy types were reused: A character model could be given half a dozen paint jobs and different attributes such as more or fewer life points, then seeded throughout areas. A brown-and-red zombie in Act One’s pastures posed less of a challenge than a blue-and-green zombie roaming the marshy shores of Act Three’s jungle. It was a marked difference from Diablo, where leaving one environment type and setting foot in another — such as level 4, the final cathedral-themed dungeon, to level 5, the first floor of the catacombs — ushered in a difficulty spike, a way of reminding players not to get complacent.
Mephisto was the most convenient boss to kill because you’re only one level away from him, and [his] levels are pretty small. That was one of those things where once everybody was playing it, we looked at the numbers and said, “Why is everybody doing this?” And it was like, “Oh, because it takes you only five minutes to find him instead of a half hour.” -David Brevik, co-founder, Blizzard North
Stieg could easily manipulate an enemy’s attributes by tweaking values in a spreadsheet. “I could essentially design all of this stuff unhindered, right down to what their actual damage and hit points would be, even without needing to see any of it on-screen, until the tuning stages when nothing could substitute for actually playing the game.”
Dialing in numbers for individual enemies was tedious work, and prone to error. Stieg might adjust the value of one enemy in a particular area but misalign the health or damage of other enemies, leading to uneven difficulty. The alternative was a global difficulty curve, modified by changing a single variable. Not only did altering one variable simplify the process of massaging the challenge posed to players in an Act, or a particular area, or the entire game, the difficulty curve could effectively be scaled as high as Stieg wanted it to go. From there, the value could be dialed in through extensive playtesting based on how powerful players should be in a given area.
“All of this called for some higher math, which one engineer, Tyler Thompson, helped me to set up. Today, I think the method I’m describing is used ubiquitously, but I think we were one of the first games to do so. In fact, looking back, it was somewhat rudimentary, compared to the spread sheeting I have done, and continue to do, since. It was definitely a breakthrough at the time.”
Gut checks, usually in the form of a tough unique monster, were another method of communicating to players that they might be too weak — or too powerful — for a new area’s challenges. “I do remember the Blood Raven quest was one of the last things we put in,” remembered Phil Shenk, character artist.
Psychology lies at the center of Diablo II’s web of systems. Clicking feels good because the audiovisual feedback — walking, attacking, picking up an item — is pleasant to perform and to behold.
Blood Raven is a unique, bow-wielding Corrupt Rogue enemy that lurks in a cemetery in the game’s first Act. She moves quickly, fires off arrows as fast as she moves, and can raise undead. Blood Raven was designed in the spirit of the Butcher, a unique monster found in Diablo’s second level. “I felt pretty strongly about that, because I loved how in the first Diablo, you’d run into the Butcher, and he’d be too tough for you. You’d get your ass kicked,” Phil continued. “You’d get down to the Skeleton King and kill him, and by that time maybe you were ready to kill the Butcher, so you’d go back and kill him, and you’d feel pretty cool, like you’d progressed.”
Giving players the option to live to fight another day evoked the spirit of Blizzard North’s design philosophy. “That’s always been our goal,” said Erich Schaefer. “Make the thing easy, make it so you don’t have to read instructions on how to play. That was always important to us: Easy to pick up and get into.”
One of the easiest-to-pick-up aspects of Diablo II was the concept of flippies, any item that flips out of a treasure chest or defeated monster before hitting the ground with a telltale clang, thud, or chime. “When I started there, the first thing that I worked on was flippies. Flippies were one of those give-them-to-the-new-guy tasks,” said Mike Dashow, character artist. “Someone’s got to do all those animations of every single weapon in Diablo II, and of course they all flip differently because an axe is [heavier] than a sword.”
Items had threefold appeal. The first was purely aesthetic. “A lot of the armor worn by heroes was incredibly detailed. Bob Steele was in charge of that, and was very meticulous,” said Anthony Rivero, character artist.
Part of Bob’s process was visiting local bookstores and libraries every week to dig up new source materials. “That was fun because at the time it was a fun learning experience. I enjoy the process of, ‘How can I do this? How can I fix this?’ It was really fun.”
Bob figured the job would keep him busy for a few months before he moved on to other work, so he took up the mantle, creating daggers, swords, clubs, axes, crowns, gloves, boots, helmets, caps, shields, and all manner of other implements.
“I volunteered to do it to get the process started and ended up working on it until I left.”
The secondary appeal of items was their variety. From inlaid jewels in crowns and pockmarked wooden shafts on battleaxes, to gleaming plate mail and wicked points at the ends of halberds and great swords, Diablo II’s items are best viewed in the inventory screen, where players see the thumbnail-sized portraits Bob painted. As other artists came on, Max and Erich assigned them to help create weapon assets to ensure a huge variety of every type of item, such as the six types of caps from leather hoods and skulls to crowns and great helms fashioned from steel.
That was always important to us: Easy to pick up and get into. -Erich Schaefer, co-founder, Blizzard North
What started as a job that nobody else had wanted became a point of pride for Bob.
“Being the prick that I am, if someone didn’t meet my quality standards for rendering or texturing, I would reject them, touch them up, or withhold approval. I went on vacation, and when I came back, one of the other artists had posted a picture of me over the checklist with Approved by Robert Steele across it. I just started laughing. I think I spit my coffee out everywhere.”
Item variety extended to categorization. There are six tiers, each a jump in quality over the last and sporting names written in colored text to help players distinguish them. Normal items, with names displayed in gray text, possess no special properties, while magic (blue) and rare (yellow) gear boast extra properties such as increased damage and health, or the capacity to leech health or energy from monsters with every hit. Items in a set, displayed in green, each exhibit additional attributes and further augment the player’s abilities when half or all items in a complete set are worn. Unique items are written in dark gold text, and are given unique names. Unique items often feature the most powerful properties.
Crafted items come in two forms. Some contain sockets into which players can insert gems, which bestow different effects depending on the type of item they’re socketed into. A ruby inserted into a shield increases resistance to fire, while fitting a ruby into a weapon adds fire damage to attacks. Gems come in five tiers, each more potent than the last, and can be upgraded using the Horadric Cube.
Pete Brevik walked up to Stieg one morning and relayed an idea he’d had while watching the Conan: The Barbarian film. He had fixated on the gems in Conan’s sword, and then suggested to Stieg that they create gems players can insert into weapons.
“It added hours of replayability to the game,” said Pete, a programmer. “It’s weird how things like that happen: You just watch a movie, and all the sudden you’re converting it into another idea and you make it real in a video game.”
The Horadric Cube was the quest item that led to the team’s animated debates over the function of body parts in Diablo II’s economy, produced the second form of crafted items. “By this time, games like Ultima Online and EverQuest had come out, and we wanted to incorporate the idea of crafting, but in a simple, non-hardcore way. This was the cube,” Stieg explained.
The Horadric Cube functioned like a cookpot. Players insert certain combinations of items, called recipes, and the Cube converts the ingredients into a special item. “After we were done with the Cube, we were like, ‘That was actually really fun, combining items. We should totally have something that makes use of the Cube after players have done this quest,’” remembered Mike Huang, technical producer.
Blizzard North’s developers baked in additional recipes to whip up handcrafted items. The catch was that recipes were never shared in-game. The developers hoped players would experiment with the Cube by mixing and matching items, then go online and share recipes with others.
“The programming resources that it took to program the Cube in the first place could be massaged just a little bit to do other transmutations as well,” Michael Huang explained. “We said, ‘Well, what are things that would be really useful to make?’ Transforming one potion into [a stronger potion], or mixing and matching different things, certain gem combinations — things like that.”
Psychology lies at the center of Diablo II’s web of systems. Clicking feels good because the audiovisual feedback — walking, attacking, picking up an item — is pleasant to perform and to behold, and because there’s a strong possibility that a defeated monster will drop an item. The item could be a gem or a quiver of arrows. It could be a bundle of throwing knives or exploding potions. Or it could be a sword — but what type of sword? Which color of text? The game generates items based on numerous factors such as players’ experience level, their location in the game, and the toughness of the monsters they’re fighting.
Diablo II’s drop rates, calculations that determine how often and of what quality items drop, were finely tuned over three years of production. If better items drop too often, players will grow numb to the steady stream of upgrades. If they don’t drop often enough, players will get frustrated — because a boss is proving too difficult to defeat with their current equipment setup, or because they’re not receiving enough loot to stay invested in clicking. Variable curves influenced virtually every possible scenario. For instance, Stieg explained, players should be able to equip most items the moment they appear, but not all of them.
Players spend hours clicking on the chance they’ll find one more item, discover one new area, or because they’re on the verge of leveling up and improving their favorite skill or learning a new one.
“Players could find an item that would carry them through several levels before they’d find a better one. This was intentional as well — getting players invested in their avatar, and their avatar’s cool equipment.”
If the drop rate is just right, players will look forward to getting a new sword, staff, helm, or pair of boots — even if it’s gear they can’t use quite yet, and even as they bond with the gear they currently have: the boots that increase their walk and run speed by twelve percent, the sword socketed with two sapphires to add cold damage and slow enemies, the chainmail socketed with three diamonds to boost all their resistances, the gloves that bump each of their skills up by one point.
Progression is another strand in the web. Every click carries players forward: to a new area, to the completion of a quest, to new items, and to leveling up. Each level-up earns them five stat points and one skill point. Players may not consciously realize it, but they’re playing for more than loot. Fans of Sid Meier’s Civilization series of strategy games know the phenomenon of one more turn: the game loop is so addictive that they can always find a reason to keep playing. Diablo II’s loop operates in a similar way. Players spend hours clicking on the chance they’ll find one more item, discover one new area, or because they’re on the verge of leveling up and improving their favorite skill or learning a new one.
“One thing that’s kind of addictive — and it’s purposely addictive because it’s what we were all looking for — it’s basically slot machines inside of slot machines inside of slot machines,” said Peter Kemmer, programmer.
The developers anticipated that players would search for the most efficient ways to farm items of the highest quality. That was fine. Waypoints, portals scattered through each Act, let players travel to an area instantly rather than on foot. The faster they can get to their favorite boss to kill it over and over again, the more they’ll want to play.
“That’s why everybody ended up doing boss runs in Act Three,” said Dave Glenn, environment artist. “Mephisto was the most convenient boss to kill because you’re only one level away from him, and [his] levels are pretty small. That was one of those things where once everybody was playing it, we looked at the numbers and said, ‘Why is everybody doing this?’ And it was like, ‘Oh, because it takes you only five minutes to find him instead of a half hour.’”
Every action in Diablo II is fun, even actions as innocuous as moving items around the inventory grid to organize gear. Every click is meaningful. Every click brings players to the precipice of something — or more than one something — exciting.
“You really have to test those systems through play,” Erich Schaefer said. “About one-third to one-half of testing comes from my personal play experience because I tend to play these games myself over and over every day, and then the rest of it is from people in the office complaining.”