Kazarov, Sengir Pureblood

Kazarov, Sengir Pureblood is a Commander card through and through — the multiplayer table means multiple opponents' creatures taking damage on every combat step and from every board-wide effect, which is the only environment where the counter trigger fires often enough to justify seven mana. In Legacy, Modern, and Pioneer it's a non-starter: too slow, too expensive, and the growth payoff requires a critical mass of damage sources that competitive 60-card formats don't provide. Vintage has the raw power to cast it but no reason to — better threats exist at every point on the curve. Oathbreaker is theoretically legal but the same structural problem applies: seven mana is a liability when games end faster than Commander.