Board Thread:General Discussion/@comment-5984859-20150820191414/@comment-25374175-20150915130136

Morro could not have been banished to the Cursed Realm by Destiny. In the episode “The Crooked Path”, we can see Morro’s body as a deceased skeleton. However, when the Anacondrai Cultists and Garmadon traveled to the Realm, they lack a skeleton visible on the ground. So, my guess is that he went the Cursed Realm after he died.

So, in order to support my theory, I would say he died from old age/was killed by someone.