The older a vampire becomes, the more they begin to take on animalistic
characteristics, (a process known as regressing). Kakistos – Greek
for "worst of the worst" – is so old that his hands and feet have
become cloven.
Kakistos arrived in Sunnydale with his accomplice, Mr Trick. They
are hot on the trail of Faith and are eager to kill the new Slayer.
Kakistos had already killed Faith’s Watcher, while she looked on,
helpless. Buffy initially suspects that Faith’s arrival in Sunnydale
at the same time as Kakistos is not a coincidence, but eventually
begins to trust her. With Buffy’s help Faith is able to confront Kakistos.
Buffy’s stake has no effect, but Faith kills the vampire when she
runs him through with a fence post!