The Brain Suckler is a Hardmode, post-Lunatic Cultist flying enemy spawned by the Nebula Pillar during the Lunar Events. It latches onto the player's head, dealing large amounts of damage at a steady rate, while also inflicting the Obstructed debuff for the remainder of time it is on the player. The debuff causes the player's screen to darken significantly, except for a small area around the player, similar to the Blackout debuff. It additionally has the effect of Bleeding, preventing all health regeneration.
If the player teleports using a Magic Mirror, Ice Mirror, or a Cell Phone while a Brain Suckler is still attached to them, the Brain Suckler will teleport with the player to the player's destination.
Unlike the Blackout and Darkness debuffs, Obstructed darkens the screen evenly, without factoring in light levels.
No matter how fast a player may move, a Brain Suckler will still catch up with them unless they teleport home at will.
Brain Sucklers pay homage to the iconic Headcrab enemy from the Half-Life series, as they attack by latching onto a player's head and disrupting their brain functions. Brain Sucklers are referred to as Nebula Headcrabs sporadically in the game's code, and in their banner prior to a patch, implying their name was changed at some point during development.
The Bestiary entry for the Brain Suckler: "These psionic, squid-like beasts levitate with celestial energy and latch onto the head of their prey, disrupting brain functions."