Kroy is a vampyre who can be found in his laboratory in the Sisterhood Sanctuary directly beneath the Crombwick Manor in the town of Slepe.
It is revealed in Sins of the Father that Kroy was responsible for the second outbreak of the Sleeper Plague, having discovered the original formula to the Elixir of Everlasting. He was then tasked by his superior, Damien Leucurte, to distribute the formula via Bloody Bracer, the town's local drink. The player then confronts Kroy after the conversation and kills him, before destroying his lab.
Kroy can be safespotted behind the two northern tables using magic and range. Head north to the two table and protect from magic to make this fight easier.
Note: Kroy will switch from magic to melee of what you're not praying. If you are praying magic he will then try to melee you if you are within melee range. If not Kroy will use magic.