Haakon, Stromgald Scourge + Morophon, the Boundless + Nameless Inversion + Umori, the Collector

Haakon, Stromgald Scourge

Morophon, the Boundless

Nameless Inversion

Umori, the Collector

How it works

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

This ingenious loop relies on a clever intersection of continuous cost reduction and graveyard-casting permissions. With Morophon, the Boundless on the battlefield naming "Knight" and Umori, the Collector naming "Tribal" (or Instant, depending on build), Nameless Inversion's mana cost is completely erased because the card's changeling ability makes it every creature type and spell type simultaneously. Because Haakon, Stromgald Scourge allows you to cast Knight spells directly from your graveyard, you can cast Nameless Inversion over and over again for free, repeatedly shrinking enemy creatures down to zero toughness until the opposing board is entirely wiped clean.