Poisoned Soul enhancement in Ninja Spy tree or
Sting of the Ninja enhancement (with 10 Action Points spent in Ninja Spy tree) or
Poisoned Darts core ability (20 Action Points spent in tree, Monk level 12, requires Shadow Veil core ability)
Magical Poison: Target takes 1d4 points of Poison damage (which scales with Melee Power) every 3 seconds for 15 seconds. The target's vulnerability to poison is increased by 5%. This ability can stack up to 20 times. On timer expiration, 5 stacks of Ninja Poison are removed at a time.
Ninja Poison is a damage-over-time effect and is one of a handful of player based poison attacks available to characters that aren't included on a weapon or as a spell.
Ninja Poison is injected on a critical hit with Sting of the Ninja, on a failed saving throw with the Poisoned Soul Ninjutsu enhancement (as part of a Touch of Despair finishing move) or from Poisoned Darts, a Level 12 Ninja Spy core ability.
Except when using Poisoned Darts (Which probably would be piercing damage anyway), Ninja Poison can only be injected by a melee weapon which naturally deals Slashing or Piercing damage, namely shortswords or kamas. If a feat such as Whirling Steel Strike, or an enhancement such as Kensai One with the Blade allows its user to be centered with a weapon type that naturally deals Slashing or Piercing damage, then it may apply Ninja Poison as well.
Ninja Poison's vulnerability effect increases the effect of untyped poisons found on weapons, other enhancements or items such as the Envenomed Blade, the Venomed Blades enhancement from the Drow racial tree or Rogue Assassin enhancements or the Quiver of Poison. The vulnerability appears to scale as well, from 5% with 1 stack, up to 20 stacks or 100% vulnerability (2x damage) to poison.
As a magical poison, Ninja Poison is effective on any enemy that is not immune to poison, including Named Monsters.
Ninja Poison's damage ticks once every 3 seconds, with up to 20 stacks possible. The poison can be also be forcibly purged at 1d20 damage per stack of Ninja Poison using the Poison Exploit Ninjitsu enhancement. Poison Exploit is magnified in damage using the Deadly Exploits enhancement to 1d30/1d40/1d50 damage per Ninja Poison stack removed (up to 20d50 damage, or a maximum of 1,000 points of Poison damage).