The Sentry, Golden Guardian

Three of white's premium keywords stacked on a 5/5 for four mana would read as a clean, unremarkable beater if the enters-the-battlefield trigger did not immediately hand your opponent a near-identical monster: The Void, a legendary 5/5 flyer with its own indestructibility and a compulsion to attack every combat it can. This is a mirror-image design, an unusual case where the ETB gift crosses the table, and the entire card hinges on the single keyword that separates the two bodies. Both are untouchable to destruction and lethal damage, both fly, both race. But The Sentry has vigilance, so it can swing and still hold the ground behind it. The Void has no such luxury: it is forced to attack each combat if able, so it cannot sit back on defense without simply refusing to tap out, and it holds no evasion edge over the body that spawned it. The drawback is engineered to lose the fight it forces: you field an attacker that also blocks, opposite a token leashed to offense. Strip the vigilance and the asymmetry collapses into a fair trade; drop the forced-attack clause and the token becomes a genuine threat rather than a compelled one. Flavor and mechanics pull the same direction, the hero who contains his own darkest self, and the design lets that duality resolve on the board rather than in the story text.