Ammy ability ideas

Mark of Karma
Effect: When activating Darkside, you may choose to activate one Mark of Karma effect rather than have the attack deal Shadow damage. Only one Mark of Karma effect may be active on a foe at a time, though subsequent activations of Mark of Karma allow you to change which Mark is active if you wish. If a foe goes for two whole rounds without being struck by a Darkside attack, any Mark of Karma effects on that foe are dispelled.

Karma of Pestilence - Choose a status effect the foe has inflicted during this battle. They suffer this status effect for the duration of Mark of Karma's effect, even if they would normally be immune.
Karma of War - When Karma of War is chosen, look at the last damage type dealt by the targeted foe - either ARM or M. ARM. While Mark of Karma is active, the foe suffers unreduceable HP loss equal to 1/5 of the final total damage dealt by their attacks dealing that type of damage.
Karma of Famine - When Karma of Famine is chosen, look at the element of the last elemental spell or ability used by the targeted foe. They gain a vulnerability to that element for the duration of Mark of Karma, overriding whatever their relation was to that element before.
Karma of Death - When the foe targeted by Karma of Death successfully knocks out a combatant, they suffer power break and magic break until the end of their next full turn.

The Knife That Severs
Effect: The Knife That Severs is a slow action useable three times a session. You must have previously struck an opponent with a physical attack to activate The Knife That Severs against them. When The Knife That Severs is resolved, perform a Darkside attack with the Piercing property that deals non-elemental rather than Shadow element damage. On a successful hit, choose one of the effects below and roll opposed Force against one of the attack's targets. On a success, the user loses an additional 25% HP and disables all abilities under the chosen effect's category for three rounds. Only one effect from The Knife That Severs may be active on a foe at a time.

The Knife That Severs Bonds - abilities gained through another being, such as pacts with Eidolons, sustained enchantment effects maintained by someone else, Twin Soul, and abilities gained by combining multiple beings into one.
The Knife That Severs Passions - abilities fueled by emotional power, such as Trance, abilities powered by hatred such as a Dark Knight's Killing Machine, and an Entertainer's soulful performances.
The Knife That Severs Thought - abilities that require specific knowledge and thought, such as operating a complex machine, mixing complicated alchemical compounds on the fly, or performing a specific arcane ritual to activate a magical enchantment. Ordinary spellcasting is not affected by The Knife That Severs Thought.

Sin-Eater
Effect: Sin-Eater may be activated when the Dark Knight fells a foe in battle by giving up their next Standard Action. The Dark Knight gains three Sin Feasts from this foe that last until the end of the encounter. Consuming a Sin Feast allows the Dark Knight to use one offensive or damaging spell or ability the foe has used against them or their allies without paying the ordinary costs. This may only be done when the copied spell or ability can normally be used and requires the same type of action. Ex: Copying a spell would require a Slow Action on the Dark Knight's turn, but a Counterattack ability may be activated on an enemy's turn in response to an attack. Sin-Eater does not allow the Dark Knight to break limits on once/session or once/battle effects.