If my opponent used eclipse on my stellacorn and that spell resolves then I use discipline, what is its might after the discipline resolves?

Its Might is 1.

Your Stellacorn Herder (3 Might) drops to -1 while Eclipse's -4 is active — Eclipse has no "minimum of 1," and units can hold negative Might for arithmetic (Rule 143.2.b). Discipline then adds its +2 as a separate layer: -1 + 2 = 1 Might. It stays that way until the "this turn" effects end at cleanup. (Negative Might is treated as 0 for combat/damage checks, but it's kept as a real value for calculations.)