They are pulsating purple blobs with horns and one eye in the middle of their bodies. They are capable of sticking to walls and floating on the poisonous fluid in the Black Comet, and are blood-suckers. They don't have much endurance, and can be killed with one hit, regardless of the weapon. In The Last Way, after the Black Arms release a paralyzing nerve gas, Black Doom sent a group of Leeches to devour the blood of Sonic, his friends, and Eggman. Black Doom reveals that the rest of the planet's population will suffer the same fate. Shadow squashes the one on him then chases after Black Doom, leaving the others as they are frozen in place and slowly drained of blood. Eventually, the effects of the gas wear off, and the heroes and Eggman escape to witness Shadow defeat Black Doom once and for all.