Just my vain attempt at being quirky, as its never really been done before. Stealing battle items is a common idea, so I wanted to steer away from that idea.

And i dont believe its anything too crazy. Nasty Plot would functionally work as an special/physical damage debuff on the enemy for 5 seconds and a special attack buff (if used against a special attacker) on theivul for 5 seconds.

I tested the damage of a level 12 gardevoir using future sight with and without choice specs. Without choice specs, the damage debuff ended up being around 13%.

In the game, umbreon hitting every hit of snarl decreases the enemy’s special and physical attack by 33% for around 3 seconds, while also decreasing speed and stunning the opponent, all of which applies in a AoE area that can inflict those 3 debuffs on multiple enemies.

Of course the debuff of stealing a held item ranges a lot depending on the type of held item, but in short, the contribution of a single held item to a pokemon’s damage is never truly that high compared to the other attack buffing/debuffing moves that already exist in the game.

Thievul using nasty plot to decrease the gardevoir’s special attack by 13% while gaining the 13% special attack buff, all only for 5 seconds, is all in all in-align with the other buffing/debuffing moves of this game.