This section is meant to be a catch-up for those who don't know anything about Isaac. If you're familiar with the game, feel free to skip to the next section.
Binding of Isaac is a roguelike, meaning every floor is randomly generated. Luckily, the flash version was made before having seeded runs was commonplace in the genre, so a lot of the random elements such as item drops can be manipulated as they come up, rather than needing to search for a perfect seed.
The moment-to-moment gameplay revolves around clearing rooms full of enemies until you reach the boss, defeating which grants you the access to the next floor. Reach the final boss and you win! Simple as that. Along the way you will collect items that help do this, anything from a basic stat upgrade to a modifier that completely changes how your tears work.
Items are split between passive and active, as you can probably figure, passive items do their job passively, simply by existing. Active items need to be activated by pressing spacebar. Once used, most will take 1-6 rooms to recharge.
There's also cards and pills, both having a special effect when consumed. Cards tend to have more temporary, one time use effects, while pills are generally stat up/downgrades.
Character Swap Glitch
Those more familiar with Isaac might have noticed I chose Cain as my character, despite him being locked. By choosing a character to start the run as, in my case Isaac since he's the only one unlocked on a fresh savefile, then backing out to the main menu, pressing start again and then hovering over a locked character, that character will get picked to start the run, regardless of if they're unlocked or not.
Animation Cancelling
Isaac loves to hold stuff above his head and not moving an inch while doing so. As you might imagine that's rather slow, so where possible I cancel the animation. There's a couple ways of doing this, but the only relevant one for this TAS is interrupting it by standing on a trapdoor when it becomes active and going down to the next floor.
Brimsnapping
The brimstone laser is ordinarily only able to hit an enemy for 2 ticks of damage per shot, however by tapping the shoot key when the laser is firing, it's possible to do up to 5 ticks of damage per single charged shot. It's a major DPS increase and pretty much singlehandedly makes brimstone the go-to item for the build.
Early Womb/Sheol
Despite the passages to these areas not spawning in their intended locations yet, they're still accessible if one can produce a trapdoor down on the correct floor. There exist one active item, We Need To Go Deeper, which can produce said trapdoor on demand, allowing me to skip entire floors and get to these locations earlier.
The Binding of Isaac is a game built on randomness. The floor layouts, the items you get, the bosses you fight, almost everything in random to some extent.
In the TAS, there's 3 relevant forms of randomness:
Item Pools
Certain items can only appear in certain rooms, this is referred to the room having a certain item pool. Some items are added to a random pool or have a chance to not appear at all. This is rolled at the start of the run and does not change.
We Need To Go Deeper, an item crucial for skipping floor and significantly speeding up the run, can appear either in the Devil Room pool, Secret Room pool or not at all. In this TAS I did not get it in either pool, but I managed to work around that (more on that in the route section).
Floor-gen
Certain aspects of the run, such as the floor layout, the specific rooms' layouts (both normal rooms with enemies as well as icon rooms like shops or item rooms) and curses are set the moment the floor is generated. Luckily the floor generation can be manipulated by setting a different system time (first floor) or by delaying entry to the next floor (floors 2 onwards). Unfortunately, manipulating later floors' generation appears to be rather limited, with layouts often repeating every couple frames. This makes hitting multiple RNG points in the floor gen very hard to manipulate, especially past the first floor.
On Demand RNG
Items on pedestals, in shops, rewards for clearing rooms and some other random effects are generated on demand, when they're needed. This form of randomness is easiest to manipulate and I make thorough use of it throughout the entire run.
The key to a fast Isaac run is twofold.
1. Damage
To win the game, you need to kill the boss. To kill the boss, you need damage. Simple as that.
For the TAS, I want to do as much damage as I can while collecting as few items as possible. Luckily, there is an item combo that does just that:
- Brimstone - replaces Isaac's tears with a blood laser that needs to be charged up, but does 2 (5 when brimsnapping) ticks of 3x base damage.
- Polyphemus - a +4 damage up as well as a massive 2x damage multiplier, at the cost of greatly increased fire delay
2. Floor Skips
All the damage in the world won't save you from needing to clear tons of rooms. However! Some items can. The shovel, We Need To Go Deeper, allows one to create a trapdoor on demand and go to the next floor right away. Of course, active items need to be recharged, but there's an item build that can work around this too.
- Dead Sea Scrolls - While not the aforementioned shovel, DSS is an active item that, when used, picks one of 26 possible effects. One of these effects is We Need To Go Deeper.
- Habit - An item from the shop that recharges active items when the player takes damage. Getting hit once gives back half of a charge.
- Scapular - Finally, to fuel the habit charges, this item will activate when the player falls to critical health and once per room will provide them with a soul heart.
Basement 1
Starting off the run, I fire tears at the wall for 2 seconds. This is to advance RNG so that the item room contains DSS, which I need for the rest of the run. After picking it up, I manipulate the effect to be Kamikaze!, which causes me to explode. I take a full heart of damage, but it doesn't matter since I'll be trading that health away later. I use the explosion to open up the secret room and wait a couple frames before entering to manipulate 3 of the 9 coins to be nickels. I need a total of 20 cents for a shopping trip later and this is the best way to get it.
From the secret room I enter the curse room containing 2 red chests. I manipulate the first one to give me 2 soul hearts, which I will need as Habit fuel later. Technically I only needed one of the two, but since I need to wait for RNG anyway, I collect both. The second one teleports me to the devil room, where I pause briefly to manipulate RNG so that the devil will offer me Brimstone for 2 heart containers. Since I got the soul hearts from the chest earlier, I can take the deal and live. First though, I manip DSS to give me the shovel effect right next to the brimstone item, so that I can pick it up and cancel part of the animation by descending down to Basement 2. I could do this since Kamikaze! has a 0 room charge, so it didn't use a DSS charge.
Cellar 2/Caves 1
I complete my build, manipulating RNG and doing animation cancels where possible.
Caves 2
I enter a nearby curse room to snag a pill for later.
Necropolis XL
Curse of the Labyrinth combines both Necropolis floors into one big one, meaning one trapdoor skips 2 floors, saving a decent chunk of time.
Additionally, I get an extra half of a DSS charge on the enemies here before leaving and manipulate the pill to give me Balls of Steel, which gives me +2 soul hearts. The enemies on the upcoming floors start dealing full hearts of damage instead of halves, so stocking up on charge and health here is crucial, otherwise I would have to clear rooms.
Sheol
Manipulated a layout 3 quick to clear rooms away from the start, then one-shot all of Satan's 3 phases because he's a chump.
