You control a nontoken Rogue, Cleric, Wizard and/or Warrior that can be cast using one colored mana.
Steps
Activate
Phyrexian Altar by sacrificing any nontoken Rogue, Cleric, Wizard and/or Warrior that can be cast using one colored mana, adding one mana of any color.
The creature dies, triggering
Mortuary, putting the creature on top of your library.
Cast the creature from the top of your library by paying its mana cost.