Activate Orthion's second ability by paying ({6} magic symbol) ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) and tapping it, creating five token copies of Priest of Urabrask with haste.
When the token Priest of Urabrasks enter the battlefield, they trigger, adding fifteen ({R} magic symbol) .
Activate Staff of Domination's third ability by paying ({3} magic symbol) and tapping it, untapping Orthion.
Activate Staff of Domination's first ability by paying ({1} magic symbol) , untapping it.
Repeat.
Once you have infinite mana, you may use it to activate Staff of Domination's other abilities.
You may also instead target any other creature using Orthion's ability.