Grelkor is spawned via a clickable skeleton in the center of the mushroom ring. While it isn't proven as cause and effect, it appears that he is summoned by killing the rotting followers of Grelkor that stand around the ring. There does not appear to be any correlation between killing them all, or killing them quickly, and Grelkor's appearance. Respawn time appears to be random. Within minutes of being killed, he was able to be summoned a second time; however, after that encore appearance, almost an hour went by without a return visit.
After killing several of his followers, the following message appeared in chat and on screen: The toxic looking bones within the circle of mushrooms begin to hum softly... Selecting the bones gives an option to disturb/activate them. This pops a non-attackable version of Grelkor, who zone-shouts FOOLS! Who dares awaken me?! He then converts to an attackable mob, who very agreeably dies and produces nice gear in a chest.