While you're the monarch, your opponents' creatures can't block you, allowing your last alliance of men and elves to knock your opponents out of the game, swiftly and cleanly
The Horror rider on this feels a little tacked on, but Ill take it
You can use the ones that appear in some Unfinity booster packs, or ones from any other set that happens to include tokens that are both Clowns and Robots
While it can't exile creatures (and noncreatures) already in the graveyard like Ooze can, Slime passively exiles anything that dies on its watch for no mana at all, also denying any death triggers (Solemn Simulacrum) in the process while growing into a real threat
Lingering Souls is also your best Cabal Therapy fodder, and part of what makes the graveyard such a powerful resource for the deck
Only then, after moving through two other zones, can you finally cast him