Activate Orthion's first ability by paying ({1} magic symbol) ({R} magic symbol) and tapping it, creating a token copy of Priest of Urabrask with haste.
When the token Priest of Urabrask enters the battlefield, it triggers, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Activate Phyrexian Altar by sacrificing the Priest of Urabrask token, adding one mana of any color.
When the Priest of Urabrask token dies, Orthion triggers, untapping itself.
Repeat.
Once you have infinite colored mana, you may instead use Orthion's second ability to make copies of any creature(s) you control.