Chaos

Chaos is the final ship playable in the base game. It is unique in that its weapon changes room by room.

Randomization
Every room, your weapon will change to a new one with two Keywords. This weapon is tied to that room. If you leave the room and return you will always get that weapon while in that room, even across different runs of a particular seed. No weapons can spawn, except for the Unique Weapon, Hand of the first. Upon picking up Hand of the First, all randomly generated weapons will be replaced by Uniques as long as you have ammo.

There is a Cartridge that only appears while playing as Chaos that increases the amount of keywords a normal weapon will have to three.

Ammo
Chaos has infinite ammo. Weapon Troves do not spawn.

Ammo works slightly differently than normal when playing as Chaos, as it is displayed as a percentage instead of a concrete number. Different weapons use different amounts of ammo per shot, with weapons that normally have less maximum ammo, such as Thunderhead, lowering the percentage more per shot than weapons such as Vulcan, which typically has a fairly high maximum ammo. When you run out of ammo, instead of losing the current weapon you use the room's weapon but stripped of all keywords. When in a Breach you instead use the Peashooter.

Weapon Troves spawn, but when broken will instead of weapons contain a choice between a +1 HP Pickup and a 50% Ammo Restore.

Story
Chaos enters the Facility at the same time as Overlord, similar to the parallel entrances of Null and D-13. Despite being the 11th Visitor, it reaches greater depths more quickly than its companion, the 10th.

On the sixth floor, it encounters the Machine and attempts to destroy it. Its attempt is unsuccessful and it merely damages it, taking immense damage in the process.

Very soon after, Overlord arrives to fight the Machine and also fails. Before Overlord receives a mortal blow, Chaos rams him out of the way, taking it for him. However, rather than being destroyed, Chaos fuses with the Machine.

When the Machine is later destroyed by Null, Chaos is, presumably, destroyed with it.

Trivia

 * Chaos is partially referenced as '???' in the selection menu, likely alluding to the same name that the Machine is sometimes referred by. This parallels Overlord's naming convention, the ship sharing the name of the boss that it eventually fuses with.