Dark Ball
Dark Balls chase Isaac, flying at a moderate pace and take increased knockback from tears. They're most often encountered when spawned by Little Horn, 
Big Horn and Blisters but can be sometimes encountered in some rare rooms in the Caves or Flooded Caves as well as Mausoleum rooms with Bishops. The Dark Balls Big Horn can spawn are larger and can either behave normally or have different behavior where they travel diagonally bouncing off walls.
Variations
Purple Ball
Purple Balls behave similarly to regular Dark Balls, but are immune to all damage. 3 of them will always spawn orbiting Rag Mega and detach to chase Isaac as the boss's HP drops. 
Cultists spawn a variant of Purple Balls that are indestructible but slowly transform into vulnerable Dark Balls.
Purple Balls can be destroyed by any instant kill effects such as 
Little Horn or 
Ace of Diamonds.
Visage Plasma
Visage Plasma are similar to the Dark Balls summoned by Big Horn and slowly float diagonally around the room bouncing off walls. If The Visage's mask is deactivated one will travel towards it to revive the mask. If there are 3 alive at once they can be absorbed by The Visage's heart to heal it and cause it to perform a special attack.
Ultra Pestilence Fly Ball
Ultra Pestilence Fly Balls behave like regular Dark Balls and float towards Isaac dealing contact damage. Each time it takes enough damage it spawns an Ultra Pestilence Fly and can spawn multiple at once if it takes a large singular amount of damage. Upon death it leaves behind another Ultra Pestilence Fly.
 The Binding of Isaac: Rebirth
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