Ugin's Conjurant enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and Weatherlight Compleated.
Resolve the Mortuary trigger, putting Ugin's Conjurant from your graveyard onto the top of your library.
Resolve the Weatherlight Compleated trigger, causing you to put a phyresis counter on it and draw a card, thus drawing Omarthis from your library.
Repeat.
This clever loop turns the inherent fragility of Ugin's Conjurant—cast for zero mana so it instantly perishes from having zero toughness—into a self-sustaining engine. Each time the Conjurant hits the graveyard, Mortuary neatly recycles it right back to the top of your library while Weatherlight Compleated happily tracks every single one of those deaths. With the Compleated accumulating phyresis counters off these continuous sacrifices, you rapidly draw through your entire deck to fuel a storm count or find a finishing blow.