An 800 foot fire demon with a sword the size of a Skyscraper called "Twilight".
It's from the Simonson era "first".
We didn't know what was happening, but for the 6 months leading up to the big bad being revealed, there was just the scenario of a smithy that you couldn't really make out forging a sword, and every time his hammer would temper the blade he was forging, the onomatopoeic sound effect legeneded was "DOOM!" ... "DOOM!" ..."DOOM!"
This teasing was leading up to issue 350 where Odin and Thor are about to make their final stand when they say "Hey Loki, we know you're not a complete asshole, so why not stop Ragnorok with us?"
And he did.
I love it when the bad guys daytrip into the world of goodygoodness.