Brain Suckler

Type	Flying Enemy
Environment	Lunar Events
AI Type	Star Cell/Brain Suckler AI
Damage	70/140/210
Max Life	330/660/990
Defense	34
KB Resist	50%/55%/60%
Banner	Brain Suckler Banner(Desktop, Console and Mobile versions)
Immune to
Debuff	Obstructed(Desktop, Console and Mobile versions)
Debuff tooltip	You can't see!
Chance	100%
Duration	Until the enemy is killed
Hurt	https://terraria.wiki.gg/wiki/File:NPC_Hit_1.wav
Killed	https://terraria.wiki.gg/wiki/File:NPC_Killed_1.wav

Desktop/Console/Mobile-Only Content: This information applies only to the Desktop, Console, and Mobile versions of Terraria.

The Obstructed debuff's screen effect.

The effects of the Obstructed debuff.

The Brain Suckler is a Hardmode, post-Lunatic Cultist flying enemy that spawns in the area of 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 the 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 natural health regeneration.^[1]

Notes

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.

Trivia

Brain Sucklers pay homage to the iconic Headcrab enemy from the first-person shooter video game series Half-Life, 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."