Can a unit go below 0 might? For example, if I have a 3 might unit, and "eclipse" was cast on it (which gives -4 might), and if I play "discipline" (+2 might spell) on it after eclipse resolves, does the unit become 1 might or 2 might?

Yes, a unit can have a negative Might value.

According to Rule 143.2.b, units can have negative Might values for calculation purposes. However, whenever the game checks "what Might are you" (such as during combat or for lethal damage), any negative Might is treated as 0.

Regarding your specific scenario with a 3 Might unit:

1. Eclipse is played and resolves, applying a -4 Might modifier.
   • Calculation: 3 (base) - 4 = -1 Might.
   • The unit now has -1 Might. For any game effect that checks its Might (like combat), it is treated as 0.
2. Discipline is played and resolves, applying a +2 Might modifier.
   • Calculation: -1 (current) + 2 = 1 Might.

After both spells have resolved, your unit will have 1 Might.

Note: This is my interpretation based on the rules regarding Might calculations (Rule 143.2.b) and the standard layering of Might modifiers. No official FAQ exists for this specific interaction.