User blog comment:Ninjana/Sensei Wu: Undead or Alive?/@comment-5149596-20121103050532

I see what you mean by the fact that it was stated that mortals couldn't enter, but I think the context implies that they just couldn't through normal means. The Ninja couldn't travel there on their own, so they used the Dragons. Even the Skulkin, who are undead, can't open a portal whenever they feel like getting back; they need to reach high speeds on their vehicles to make the transition!

In the mini movie Return to the Fire Temple, the Fire Dragon alerted Kai and Nya to a hidden entrance to the Underworld that the Skulkin were using to easily access Ninjago. It can be assumed then that there are certain locations in Ninjago that connect the physical world with the Underworld, and the lava waterwall Sensei Wu used was probably one of them! So, I really don't think he needed to die to get there. c: