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 Orthion by paying ({3} magic symbol) and untapping it, giving it +2/+2 until end of turn.
Repeat.
Once you have infinite mana, you may target any other creature using Orthion's ability.