Sacks are spider-web variants of normal Boils.
They cannot replace Boils or Guts, but can commonly be found in the Cellar.
They are swollen lumps made out entirely of cobwebs.
Sacks don't shoot projectiles, instead, they periodically spawn Spiders when fully swollen. Therefore, damaging them will prevent them from spawning Spiders, making them easy to kill.
- They can be spawned by spider-type bosses, such as the Widow.
- They appear in the Wrath of the Lamb DLC for the original game.