Loki

Heimdall with still say nothing, but he will strike, and strike hard, his sword crashing against Loki's armour, and Loki will counter, and Loki will attack with fierceness and intelligence and glee.
As they fight, they will remember a time they battled long ago, when the world was simpler. They had fought in animal form, transformed into seals, competing to obtain the necklace of the Brisings: Loki had stolen it from Freya at Odin's request, and Heimdall had retrieved it.
Loki never forgets an insult.
They will fight, and slash and stab and hack at each other.
They will fight, and they will fall, Heimdall and Loki, fall beside each other, each mortally wounded.