Tainted Monsters are a gameplay element featured in Ni no Kuni II: Revenant Kingdom. There are a total of 60 tainted monsters although initially only the first 50 are available.
Tainted Monsters are monsters that have been infected by an overwhelming amount of darkness. They are surrounded by a dark purple ring and the player is prompted if they want to fight before starting the battle. Tainted Monsters are optional and provide a massive increase in experience points, guilders, and other unique rewards. There are also cumulative rewards for defeating tainted monsters (i.e defeating 2 earns the player a Bulwark Brooch).
Tainted Monsters are scattered all over the world, and can be anywhere on the world map including caves forests, and shrines. The Tainted Monster journal provides the name of the location but the player must discover it on their own.
Some monsters may only become available by accepting side quests.
The Tainted Monsters are rated on a scale of one to ten stars. The more stars they have, the more difficult the fight.