The evil cousin to Elric of Melnibone. Their hard fights are described very thoroughly in the saga written by Michael Moorcock.
The background of Yyrkoon is that he is the cousin to Elric and plots to take over the throne from Elric. He's the evil counterpart to Elric in the Melniboné saga and thus accordingly stresses strength, raw power and a more conservative approach than Elric.
Yyrkoon uses the runeblade Mournblade, brought to Melniboné centuries before Elric & Yyrkoon were born. The other runeblade is Stormbringer which Elric carries with him.