This item belongs to the Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.
Notes
While Psy Fly is an effective means of projectile protection, it is not immunity, and recklessly wading into a barrage of shots can easily result in taking damage.
Psy Fly will block most types of projectiles, not just red bullets. This includes the fires from Mega Satan phase 2, the bone projectiles Bonies throw, Ipecac shots from Gurgles and similar enemies, or even the rocks that the Kineti throws at you.
This item originates from the Antibirth mod, but it was unobtainable due to a bug. It also had two scrapped abilities that made Attack Flies turn into Blue Flies when encountered and blocked tears home in on enemies. Psy Fly was originally unlocked by completing Isaac's Awakening (challenge #39).
Gallery
An unused unlock screen for Psy Fly. In Antibirth, this item was intended to be unlockable, but the unlock requirement was bugged.