Brain Suckler

Brain Sucklers spawn around the Nebula Pillar during the Lunar Events, so they only exist after the Lunatic Cultist is dead and only while that pillar stands. Nothing summons them and you will not meet one anywhere else. Their spawn rate was cut hard in a later patch, so older accounts overstate how constant they are.

It flies at your head and stays there, chewing at a steady rate while the screen darkens to a small circle around you. That darkening is flat rather than a lighting effect, so torches and light pets do nothing about it. It also holds regeneration shut the way bleeding does. Around the pillar, with fire from every direction, losing sight past a few tiles is worse than the damage. The name is a nod to the headcrab from Half-Life, and the game code still calls it that in places.

• No movement setup in the game outpaces one. It closes the gap back up whatever you are wearing.
• Mirroring out does not shake it. It rides along and is still attached when you land at your spawn point.
• Kill it on approach instead. Anything with reach works, and free minions will pull one off you.