Mindwrack Harpy

The symmetry is the whole design problem. A repeating three-card mill on every one of your combat steps sounds like an engine, but it fills both players' graveyards equally out of their libraries, and self-mill decks care about which yard grows. That symmetry is exactly why this creature belongs to a specific kind of black graveyard shell rather than a generic mill plan: it wants a deck built to convert its own milled cards into resources (delve payoffs, recursion, escape-style costs that read the graveyard as fuel) so that the "each player" clause reads as an asset to you and mere card-shedding to the opponent. The flying 3/2 body is the tempo that keeps it relevant when the mill never matters; the recurring self-mill justifies choosing it ahead of a cleaner evasive threat. Because the trigger resolves as combat opens, before attacks are declared, the three cards feed whatever you plan to cast or recur that same turn rather than sitting until your next upkeep, and that timing turns the mill into setup for the very attack it precedes. It is a build-around wearing the body of a fine roleplayer, and the enchantment type gives it a second life in decks that count permanent types for their own reasons.