Official Riftbound ruling

Does a unit with 0 or less Might automatically die?

No, units can stick around on the board with 0 (or less!) Might. They will die if they take any damage, but “0” is not a valid amount of damage, so it takes 1 damage to kill a unit with 0 or less Might. We realize that this is a little weird, but it’s important to us that cards like Frigid Touch can’t be used to actually kill units, only weaken them.

Riot's question and answer, word for word, from theRiftbound Spiritforged FAQ

The card

  • Frigid Touch
    Frigid Touch

    Spell

    Reaction (Play any time, even before spells and abilities resolve.)

    Repeat 2 Energy2 Energy (You may pay the additional cost to repeat this spell's effect.)

    Give a unit -2 MightMight this turn.