If Mossbridge Troll would be destroyed, regenerate it. Tap any number of untapped creatures you control other than Mossbridge Troll with total power 10 or greater: Mossbridge Troll gets +20/+20 until end of turn.
2008-05-01
The first ability essentially means that Mossbridge Troll always has a regeneration shield. It retains that shield even if it's used.
2008-05-01
The total power of the creatures you tap is checked only when you pay the cost. It doesn't matter if their power decreases, or if any of them have left the battlefield, by the time the ability resolves. Note that you may tap a Mossbridge Troll to help pay the cost of a different Mossbridge Troll's ability.