Note: Scolex is invulnerable to damage. Only his tail, which constantly blinks, can be damaged.
Jumps out of the ground towards the player, then burrows back into the ground. This attack is repeated multiple times. If Scolex is hit by any explosion, he will be deflected back from the explosion when in midair.
When Scolex's head pops out, he will shoot multiple blood shots of various sizes, similar to Monstro.
When Scolex's tail pops out, he will fire three Ipecac shots at the player.
Bogue ! When Scolex is slowed, some of his body parts may detach or won't even show up - they will only be visible while entering the ground (especially the tail).
Bogue ! If the final vulnerable segment of Scolex's tail is destroyed using the Chaos Card, the boss will glitch and become harder to kill. The tail will appear again if his head got bombed or he hits Isaac.
Bogue ! If you petrify Scolex (with Mom's Contacts or other similar items) when it is in mid-air, you can damage any segment of its body.
Bogue ! If you fire immediately upon entering the Boss Room, rarely, you can damage Scolex before it emerges.
Bogue ! Rarely, Scolex's sprite will be replaced with that of a second form Frail, who has an upside-down tail. This is purely a visual bug and the Frail will have all of Scolex's attacks and AI.