What does Mighty mean?

A unit is Mighty if it has 5 or more Might. Units donʼt have the keyword “Mightyˮ—as long as a unit has 5 or more Might, the unit is Mighty. When you play a unit with 5 or more Might, you can exhaust your Relentless Storm Legend to channel a rune exhausted.

Related: Rule 708, Volibear, Relentless Storm