User blog comment:OverlordRocks/Why did you guys get rid of all the pages in the immortal beings catogory?/@comment-27507964-20160205135049

There is confirmation ghosts are immortal. When Clouse was a ghost with Nadakhan he specifically said, "I wish to be mortal again." I don't know what else that could refer to.

We have never seen a single Skulkin die. In the pilots you see the ninja beat the Skulkin to a pulp and they are still alive. They lose heads, arms, legs, and they still continue.

For reference, we see young Wu at the same time period as the Grundle, which was over 200 years ago. A short while after that Garmadon tries to take the golden weapons and gets banished to the underworld. In the underworld he meets Samukai and they refer to a previous meeting. Then, 200 years later, Samukai is still around. He hasn't changed a bit.