Lying in the scarred remains of Asgard lost, he found tears he thought he'd spent long ago. It was all over, now. The world he once reluctantly called home lay in ruins and now he lay among them, bleeding all that was left of himself into the cracked earth. There was no more he could do. Perhaps that was why he cried.
He tried to lose himself in the darkened sky. The moon shone blood red and the sun was black as pitch. He was not expecting his brother, his executioner, to appear on the edge of his fading vision. Thor shouted down at him, but not in anger. Sorrow filled his eyes, words of apology pouring from his mouth. They turned Loki sick with fury.
The knife slipped so easily through his brother's ribs. Loki would have laughed at the dumb expression in the great god's face if his mouth weren't already filled with his own blood. Instead he burbled and choked, undignified and messy, his grin a cruel red sickle as the color drained from the Thunderer's flesh and his face contorted in agony. The poison in his blood must burn, he thought.
The great serpent's venom did its work quickly. Thor fell across his brother, dead. He was heavy upon Loki's broken body, but not quite so heavy as the hammer that broke him. Loki, who had not the strength to crawl away, turned back to the sky and let his fingers slip from his poisoned blade as his mind slipped into darkness.
Let that be his last mark upon Asgard.
I'm not sure where that came from. And just kidding, Thor and Loki don't really kill each other at Ragnarök. Technically Thor dies fighting Jörmungand so, that's why the poisoned dagger is present. This is probably the closest thing to fanfiction I will ever write for the Marvel characters, haha… although, I do write my own story based on Norse mythology. So there's that. ;D