What it does
Merciless guarantees a critical hit on every attack against a poisoned or badly poisoned target. A critical hit deals 1.5× damage and ignores positive Defense or Special Defense stat stages on the target (though it does not ignore negative ones). The effect applies to every attack, every turn, as long as the target remains poisoned. Non-poisoned targets are unaffected — this is not a global crit boost.
Who runs it
Toxapex is the carrier at 12% usage. Its Water/Poison typing gives it access to poison-inflicting moves. Baneful Bunker is its signature protect variant — any Pokémon that hits Toxapex with a contact move during Baneful Bunker gets Poison, setting up the Merciless guaranteed crit on the following turn. Toxic is the direct alternative for applying Poison immediately. The hard constraint is Toxapex's 63 base Attack and 63 base Special Attack: even with guaranteed crits, the damage output stays modest. That ceiling is why 88% of Toxapex run Regenerator instead.
How to use it
The core sequence: inflict Poison via Baneful Bunker or Toxic, then every subsequent attack deals crit damage. Scald is a usable Special Attack option with a 30% burn chance, but Burn does not activate Merciless — only poison does. Scald's value here is the incidental burn rather than any synergy with the ability. Baneful Bunker's poison proc requires the opponent to use a contact move, so it only sets up Merciless against physical contact attackers.
Tips & strategy
The main reason Merciless sits at 12% is that Regenerator is more broadly useful. Regenerator restores one-third of Toxapex's max HP when it switches out, letting it cycle in and out for sustained chip damage and Baneful Bunker turns regardless of whether the opponent is poisoned. Merciless requires the poison setup to function and does nothing if the opponent carries a Lum Berry, uses a Pokémon immune to poison (Steel types are completely immune, Poison types are also immune), or switches out to remove the status. The setup dependency and Toxapex's low offensive stats make this a niche pick that rewards patient, setup-heavy teams.