Toth
From Multiversal Omnipedia
Toth was a demon, last member of the Tothric Clan. Unlike most demons, Toth chose to battle using devices and tools rather than through brute force. He came to Sunnydale and attempted to defeat Buffy Summers, by using a magical device that would split her into two beings- one being her stronger "Slayer" half and the other her weaker "Buffy" half. Toth then intended to kill the weaker half so that both would die. However, Toth missed Buffy and accidentally hit Xander Harris, splitting him into his weaker and stronger halves instead. He eventually waited for another opportunity to strike, and did so at Xander's new apartment, but Buffy slew him.