Cast Nameless Inversion from your graveyard, as it is a Knight card, by paying {0} due to The Ur-Dragon and Morophon, giving a creature +3/-3 until end of turn.
If the creature has toughness 0 or less, the creature dies as a state-based action.
Repeat as desired.

Reduce the toughness of creatures opponents control to 0

Reduce the toughness of each creature opponents control to 0 upon entering

The Ur-Dragon

Haakon, Stromgald Scourge

Morophon, the Boundless

Nameless Inversion

Cast Writhing Chrysalis by paying {2}{R}{G}. Ulalek and Writhing Chrysalis' first ability trigger.
Holding priority, cast Nameless Inversion by paying {1}{B}
Ulalek triggers, causing you to pay {C}{C} to copy the Writhing Chrysalis spell, Ulalek's trigger and Writhing Chrysalis' trigger.
Resolve the copy of Writhing Chrysalis' trigger, creating two 0/1 Eldrazi Spawn creature tokens.
Activate both Eldrazi Spawn creature tokens by sacrificing it, adding {C}{C}.
Resolve the copy of Ulalek's trigger from step 4, causing you to pay {C}{C} to copy the Writhing Chrysalis spell, Ulalek's trigger and Writhing Chrysalis' trigger.
Repeat from step 4.

Infinite creature LTB

Infinite creature ETB

Infinite creature sacrifice triggers

Infinite death triggers

Infinite creature tokens

Infinite copies of abilities you control on the stack

Ulalek, Fused Atrocity

Writhing Chrysalis

Cast Drowner of Truth by paying {4}{C}{G/U}{G/U}. Drowner of Truth and Ulalek trigger.
Holding priority, cast Nameless Inversion by paying {1}{B}, triggering Ulalek.
Resolve the Ulalek trigger, causing you to pay {C}{C} to copy all spells you control and then copy all activated and triggered abilities you control, ensuring to put Drowner of Truth's trigger on top of Ulalek's trigger.
Resolve the copy of Drowner of Truth's trigger, creating two 0/1 Eldrazi Spawn creature tokens.
Activate the Eldrazi Spawns by sacrificing them, adding {C}{C}.
Resolve the copy of Ulalek's trigger, causing you to pay {C}{C} to copy all spells you control and then copy all activated and triggered abilities you control, ensuring to put Drowner of Truth's trigger on top of Ulalek's trigger.
Repeat from step 4.

Infinite magecraft triggers

Drowner of Truth // Drowned Jungle

Cast Nameless Inversion from your graveyard, as it is a Knight card, by paying {0} due to Temur Battlecrier and Morophon, giving a creature +3/-3 until end of turn.
If the creature has toughness 0 or less, the creature dies as a state-based action.
Repeat as desired.

Temur Battlecrier

Combos with "Nameless Inversion"