He's talking about Ragnaok - the twilight of the gods in Norse mythology. Surtr, if I remember right, was a fire giant ? When a new volcanic island was formed off Iceland back in the 1960s, it was named for him - Surtsey. http://www.surtsey.is/index_eng.htm Actually, Thor also killed Surtr, so it was actually two opposing powers negating each other. This happened right across Ragnorak - gods were killed, but also killed their killers [if you see what I mean...]. But a number of gods, including Thor's sons, survived and brought a new order to the world. . .
The Twilight of the Gods is a prophecy. Thor ain't dead yet. Ragnarok is always not now but will happen soon, and always will be.
there is this thing called definitions. latin prefixes, like "im" have very specific non-ambiguous ones.
One of the definitions of immortal in my dictionary - and the one I'm going with here - says: A person who is remembered enduringly. So you can be dead and still by immortal if your memory lives on. William Shakespeare, was/is immortal, Plato, Confucious, Malcolm X, Elvis Presley, Jesus, Judas, hell - even Adolf Hitler could be considered immortal in a sense. To the best of my knowledge they're all dead [with the possible exception of Elvis] and could all be considered immortal. In fact, immortal because they're dead. Same thing with gods - they strut their aeon on the stage, then fade away. Remember Ozymandias ? No ? Well, that proves my point. As far as Thor goes, though, I have reason to believe he's not dead. Semi-retired possibly, but not dead. .