If Mirri the Cursed is blocking or blocked by a creature that doesn't have first strike or double strike, Mirri will deal combat damage to that creature, then get a +1/+1 counter, then (if the other creature survived) Mirri will be dealt combat damage.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.