A group of four 86^ heroic topiaric rippers will spawn every 20% of Riyadh's health, and he will go back to his spot. When the adds spawn, he puts a debuff on the group that prevents hostile spells. Kill the adds, then continue killing Riyadh. He also dispels your buffs. This fight uses up a lot of mana.
Riyadh has approximately 2,400,000 HP himself and the adds he spawns have another 3,500,000 HP combined. Each wave that spawns contains 4 topiaric rippers (void beasts) and each one has approximately 175,000 HP.