Poison Headcrabs are a variant of the standard Xen Headcrab that appear in Half-Life 2. These creatures are easily distinguishable from their cousins, having rot-black skin and longer legs that are covered in thin hairs. As the name suggests, they are highly poisonous and deliver a devastating neurotoxin to their prey. Like the other headcrab types, these were weaponized by the Combine.
Poison headcrabs normally move slower than standard headcrabs and make an unnerving chittering sound as they hunt. When a potential host draws near, they will emit a mouse-like squeal before pouncing with alarming speed at a victim. Like the other headcrab variants, they will convert victims into zombies. To ensure that their prey does not struggle, the poison headcrabs' mandibles inject a powerful neurotoxin into the blood of their victims. A victim will then be swiftly placed into a death-like state, allowing the headcrab to fulfill its assimilation more easily.
Poison headcrabs can easily blend into dark areas and their cries can badly demoralize anyone moving through their territory. They move slowly initially, but can leap farther and faster than standard headcrabs. Any firearm can be useful for dispatching these creatures, so long as the shots count. Poison headcrabs have greater survival instinct than their cousins and will quickly flee if attacked.
Advanced protective gear and medical kits are essential when fighting poison headcrabs. During Half-Life 2, Gordon Freeman's HEV suit protected him from the creatures' neurotoxin, but only to a degree. Being bitten by a poison headcrab dropped his health to 1 and while the suit administered antidotes that slowly restored him, he was extremely vulnerable to attacks from other forces while recovering health.