Melee is a combat mechanic that appears on creatures. Whenever a creature with Melee attacks, it gets +1/+1 until the end of the turn for each different opponent being attacked that turn. So in one-on-one Magic, this will always be +1/+1. In a four-player Commander game, this could be +3/+3 if you send attackers at each opponent.
It is possible for a creature to get multiple instances of melee, and those bonuses do stack. The bonus applies once all attackers have been declared, before blockers are assigned, so the defenders will know exactly how big the creatures they’re blocking are.
That’s it really. This is a simple one, clearly optimized for multiplayer formats. As such, it’s unlikely to ever see print in a Standard-legal set, but it’s shown up again here and there in Commander products after its initial appearance in Conspiracy: Take the Crown. There’s no reason to think it won’t still continue to show up in places like that every now and then.
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