When Digsite Conservator dies, it, Mortuary and Sifter of Skulls trigger.
Resolve the Sifter of Skulls trigger, creating a 1/1 Eldrazi Scion creature token.
Activate Ashnod's Altar by sacrificing the Eldrazi Scion, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Mortuary trigger, putting Digsite Conservator on top of your library from your graveyard.
Resolve the Digsite Conservator trigger, causing you to pay ({4} magic symbol) to exile Digsite Conservator from the top of your library and cast it without paying its mana cost.