It's a mystical compound -- the Latin name translates roughly as "Killer of the Dead." Used on vampires.
Willow Rosenberg[src]

Angel is struck by an arrow dipped with the poison.

Killer of the Dead (or Interfector Mortis in Latin) was a mystical poison capable of killing vampires when their bloodstream was infected with it, causing a severe and painful fever in the process. The only cure known was the blood of a Slayer. Only two vampires had survived this poison, the second being Angel, who was cured by the blood of Buffy Summers after being infected with a poisoned arrow by Faith Lehane (Buffy had attempted to capture Faith to use her blood, but Faith escaped by falling on a passing truck after she was stabbed).