Activate Shilgengar by sacrificing the Goblin, Orc or Army
When the Goblin, Orc or Army dies, Pitiless Plunderer, The Great Goblin's second ability and Mortuary trigger.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Mortuary trigger, putting the Goblin, Orc or Army onto the top of your library from your graveyard.
Resolve the Great Goblin trigger, exiling the Goblin, Orc or Army from the top of your library with the ability to play it until the end of your next turn.
Resolve the Shilgengar ability
Activate the Treasure by tapping and sacrificing it, adding one mana of any color.
Cast the Goblin, Orc or Army from exile by paying its mana cost.