I don't have the game on hand right now, but I can guess that if 2 to the power of 16 equals 32768 (meaning 32767 if starting from zero), then if the spirits are in the same unit, something bad will happen after attacking a single peasant. The game probably doesn't allow having more than 32767 units in a unit... so, after the attack, the number of specters should increase by +1... but here, the unit counter probably either resets to zero or goes into the negative.

And on the battlefield, either a single peasant (the second unit) will remain, or 32767 specters will switch to your side (the second option is unlikely).