"Thor Could Be About to Lose His Most Trusted Ally." - "No, not Heimdall."
Actually, there are two varieties in Norse Mythology. After Heimdall gives a signal with his horn that giants came to kill Asgardian gods, he ends up in a 1v1 battle against Loki. One story depicts Loki being beheaded by Heimdall as fires of war consume Asgard. Other "story" says they both die in their battle. Altough, I don't believe any of this will happen in next Thor movie. In my opinion, it's just a bad choice for a movie to be named "Ragnarok" as it brings up all the "main characters" maybe getting killed. That would certainly make it very difficult to make another movie after (if translating Ragnarok literally) "the end of the world". Honestly, I hope movie isn't named Ragnarok with a purpose. I believe movie fans wouldn't like this movie to turn out being the last one.