Creates a large red aura around herself, then begins to fire rings of bullets which have openings in Isaac's direction. These bullets split when they leave the aura.
Sings, charming your familiars and turning them against you. You can cancel this by attacking them.
Fires music notes in hexagonal directions, which at first travel in a straight line away from her, but then begin to curve either to the left or right.
Spawns 3 Incubus-like familiars which rotate around her, occasionally firing tears at Isaac.
Phases into the ground and chases Isaac, leaving a trail of creep.
Her ability to charm familiars can be frustrating if the player has Dead Cat, because if she is defeated while it is charmed she'll revive with 10% health, stealing one life of the Dead Cat in the process.
Other familiars that give more lives to the player will also revive her.
After the battle is over, The Siren's skull will lie on the floor. Breaking the skull will unlock the Song of the Siren.
Sirens are monsters found mostly in Greek mythology, and are usually portrayed as women with the bodies of birds. They would roost near shallow rocks and sing their hypnotizing song, trying to lure sailors to their doom.
Her "ring" attack in which tears emit from her and split once far away is almost identical to that of the Gorgun's, a boss from the game Enter the Gungeon.
The Siren was not in the initial version of Antibirth, but was patched in about a day after release.