Power Thief (05577mou)
Stat-lowering effects that the Pokémon inflicts raise its own stats.
When this Pokemon lowers another Pokemon's stats, this Pokemon's stat is raised by the same number of stages.
Be careful assigning this Ability, as it's quite strong. Best on things that are frail so that there is risk going for your initial boosts.
This Ability can be found in this Collection:
Altera.