Nighthowler

Bestow is the wrinkle here, and it's load-bearing in a way the base creature can't be: a 0/0 with no static body of its own is unplayable as a creature unless something feeds it, so the card hands you two ways to deploy the same scaling threat depending on which one survives. Cast it as a creature and it's an enchantment that grows with every dead creature on the table, yours and theirs alike; cast it for its bestow cost and it pours that same graveyard-counted bonus onto a body you already trust, then survives that creature's death by reverting to a creature itself. That second mode is the whole point of bestow as a mechanic, and Nighthowler is one of its cleanest demonstrations: an Aura that refuses to be two-for-one'd, because removing the host leaves you with the enchantment-creature standing where the Aura was.

The other half of the design is that the +X/+X reads from all graveyards, not just yours, which quietly turns the card's growth into a shared resource. Every trade, every chump block, every sweeper that fills both yards pushes X higher, so the longer a grindy game runs the larger Nighthowler gets without you spending a thing. It's a self-fueling threat in a slow attrition fight and a curve-out liability in a fast one, and the bestow option is the lever that lets you choose which game you're playing.