If I use Moonfall on my opponent's 1 might unit, does it go to 0 or -1?

If you use Moonfall on an opponent's unit with 1 Might, its Might will become -1.

According to Rule 143.2.b, a unit's Might can mathematically go below 0. While the game treats its Might as 0 for interactive purposes (like combat or checking for lethal damage), the actual Might value for calculation purposes is -1.

If you were to subsequently apply a positive Might buff to that unit, it would be added to that -1 value (e.g., a +2 Might buff would result in 1 Might: -1 + 2 = 1).