Farrel's Mantle

If you put this on your opponent’s creature, they decide whether or not to make their creature deal no damage to you.

If the same creature is enchanted with more than one Farrel’s Mantle, they’ll each trigger if that creature attacks and isn’t blocked. The creature’s controller may have it deal damage multiple times. As long as the enchanted creature’s controller chooses to use Farrel’s Mantle’s ability at least once to deal damage, the enchanted creature will deal no combat damage.

If the enchanted creature’s controller chooses to have it deal no combat damage, that combat damage is not prevented. It’s simply not dealt. A creature whose damage can’t be prevented (such as Excruciator) would deal no combat damage this way.