After obtaining 3 fly items from the following list ( 3 items with the fly item tag), Isaac turns into a giant humanoid fly and gains flight. While in this form, most types of enemy flies will automatically be converted into Blue Flies. Enemies such as Boom Flies and its variants will become friendly and be brought with Isaac to other rooms and floors.
Increases the size of Blue Spiders and Blue Flies and doubles the amount of damage they deal. Also increases the effectiveness of fly and spider familiars.
Spawns an orbital familiar that blocks enemy shots. Upon taking damage, the familiar will pursue enemies for the rest of the room, dealing 22.5 damage per second.
Spawns fly familiar that moves in a straight line and circles around the first wall or obstacle it hits, dealing 105 damage per second on contact. Spawns fly familiar that moves in a straight line and circles around the first wall or obstacle it hits, dealing 56 damage per second and forcing nearby enemies to attack it.
Beelzebub, also known by "Lord of the Flies" and many other names, is known as one of the 7 princes of hell. Beelzebub is also another name for the devil.
In modern Catholic tradition, Beelzebub is regarded as one of the 7 princes of Hell; however, the origins of the name Beelzebub actually dates back to an early Jewish insult for worshippers of a Canaanite Deity Ba'al 'Lord', or Ba'al-Zvul 'Lord of the manor/dwelling', calling him Ba'al-Zvuv, 'Lord of the flies'.
The achievement 'Lord of the Flies' is a reference to this.