It was engraved with a language that was tied to the same language on the Seal of Danzalthar (proto-Tawarick). The inscription read "The blood which I spill I consecrate to the oldest evil". The First Evil in the form of Warren told Andrew to "Drive the words deep into [Jonathan]".
After the murder Andrew washed off the blood and put the knife in Buffy Summers' cutlery drawer because she didn't have any steak knives. Buffy later used it to threaten Andrew to get him to confess to the murder and spill tears so the Seal would close.
Behind the Scenes
This knife was purchased from a catalog as were many of the knives and weapons on the show. This knife, however, served double duty, also appearing as the Dagger of Lex in "Life Serial" in Season 6. The handle was painted in sections with silver paint to make it appear as mother of pearl.