Fistula's appearance is extremely grotesque-looking, made of pieces of rotten flesh (with red fleshy skins from the inside) sewn together with thick stitches to form a large sphere. It sports with several hair strings, warts, and holes. In Edmund's artwork (and including the one from Rebirth), a few Chargers can be seen peeking out from the holes.
Total HP: 60 (Large) + 4x15(Medium) +8x8(Small) = 184
It resembles Envy in how it behaves, beginning as a single entity before splitting into four smaller entities upon taking damage, which then each split into two with further damage. However, unlike Envy, when reduced to the lowest size, the sections become Chargers.
The smaller a piece becomes, the faster it moves around. Pieces can also collide with each other, which changes their direction. Because of this, it is recommended to leave bigger parts intact for as long as possible, and focus on one part at a time. That way, the movement of the parts is more predictable, making the parts easier to dodge. It is also advised to kill the Charger before moving onto the next part, if possible.
Despite floating in the air, Fistula and its parts cannot move over Rocks. However, as of the Wrath of the Lamb expansion, it can move over Pits.
In this form, it will initially split into three entities instead of four and will spawn Boom Flies and Red Boom Flies on death. This version of Fistula has slightly more health, is larger, and moves slower.
Every time it splits, it shoots 8-way bullets. It also spawns Eternal Spitties on death.
As a miniboss, Fistula will not spawn a Charger or Boom Fly if the smallest part is killed above a pit.
From the artbook, Chargers seem to peek out of the holes while Fistula's smaller parts move around.
Fistula appeared in the demo from the original game.
The tradition of fighting the boss with its parts breaking into smaller and smaller pieces is based on the arcade classic Asteroids.
A fistula is a medical condition in which organs and other bodily tissues are abnormally connected to each other.
The name "Fistula" is used for another enemy in another game by Edmund McMillen: a flash game called The C-Word on Newgrounds.
It's also notable that in the background of Host, another one of Edmund's games, there is what appears to be a Fistula.
Fistula also slightly resembles a dead Dumpling, a creature from his Badlands series.
In older versions, Fistula used to shake while moving. Since the Wrath of the Lamb update, it no longer does so.
Using Mom's Bra on the first form can cause subsequent forms to get stuck behind rocks, making this boss unbeatable if the player cannot break the rocks. The same is true with Teratoma.
Using Mom's Pad on any form may cause it to become trapped in rocks making it impossible to defeat without destroying the rocks. The same is true about Teratoma.