Omarthis enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and High-Society Hunter.
Resolve the Mortuary trigger, putting Omarthis from your graveyard onto the top of your library.
Resolve the High-Society Hunter trigger, causing you to draw a card, thus drawing Omarthis from your library.
Repeat.
This clever loop uses Mortuary as the ultimate safety net, ensuring that whenever a creature hits your graveyard, it immediately returns to the top of your library instead. As Omarthis, Ghostfire Initiate enters the battlefield with zero toughness, it instantly perishes, sending itself right into Mortuary's waiting arms. This death simultaneously feeds High-Society Hunter, whose tragic appetite forces you to draw a card—thus immediately pulling Omarthis back into your hand so you can cast it again for zero mana.