Activating Cudgel Troll’s ability causes a “regeneration shield” to be created for it. The next time Cudgel Troll would be destroyed that turn, the regeneration shield is used up instead. This works only if Cudgel Troll is dealt lethal damage, dealt damage from a source with deathtouch, or affected by a spell or ability that says to “destroy” it. Other effects that cause Cudgel Troll to be put into the graveyard (such as reducing its toughness to 0 or sacrificing it) don’t destroy it, so regeneration won’t save it. If it hasn’t been used, the regeneration shield goes away as the turn ends.

If Cudgel Troll is dealt lethal damage and is dealt damage by a source with deathtouch during the same combat damage step, a single regeneration shield will save it.

To work, the regeneration shield must be created before Cudgel Troll is destroyed. This usually means activating its ability during the declare blockers step, or in response to a spell or ability that would destroy it.