In the pilot episodes Sensei Wu goes down a lava fall to go down to the underworld. No mortal can go down there. The only other way is to ride a dragon. So Sensei Wu had to, well, commit suicide. Undead in Ninjago equals someone who was once dead but actively still participates in the episodes such as Garmadon and the undead pirates. So Sensei Wu died... but he's still alive in the sense he's back in the real world? So doesn't that mean he qualify's for undead? Please let me know your thoughts if I'm correct or not.
Chop chop! -Ninjana