Whenever Mirri deals combat damage to a creature, put a +1/+1 counter on Mirri.
Mirri the Cursed is a mono-black voltron commander with built-in evasion (flying), first strike, and haste, so she comes down on turn 4 swinging immediately. The plan is to suit her up with equipment and auras, force chip damage and card advantage, and turn each combat into a snowballing threat that closes games with commander damage. You play a low-curve, fast aggressive game backed by black's removal and reanimation to protect your investment.
Native flying, first strike, and haste make her an immediate, resilient evasive threat the turn she lands
First strike plus the counter trigger means she grows whenever she connects with blockers and survives most combats
Mono-black gives access to the best targeted removal, tutors, and recursion to clear blockers and rebuild after wraths
Cheap to cast and easy to make lethal fast with a focused Voltron build
Single-creature strategy is extremely vulnerable to removal and gets blown out by board wipes
Commander tax makes repeated recastings punishing in a glass-cannon shell
No color access to counterspells, protection from removal beyond black's limited toolkit, or graveyard hate
The counter trigger only fires on damage to creatures, so it does nothing against open boards
Legendary Creature — Vampire Cat
