Dead Inside

Certain creatures exhibit foul manifestations of pestilence and putrescence. They might necrotize flesh with a glance, command creeping horrors that bite and gnaw, or possess mutated guts that burrow through living flesh with twisted maws.

I Active The creature grapples the character, forcing the victim to make a [Strong←Strong] test, or otherwise spend an Action pinned and helpless, without itself losing the ability to make an Active Action in future rounds (i.e. it can continue to attack other targets normally or remain engaged with the victim, tearing and biting)
II Active As above, but the creature does more than pin the character if they fail their [Strong←Strong] test. The creature plants some foul putrescence into the target’s flesh, that gnaws, bites and consumes. The victim suffers 2 Penetrating damage per turn as their innards putrefy, liquefy, or are consumed. The experience warrants a Hard [Resolute] test to avoid Despair on the first round of suffering damage. Thereafter, the character must succeed in a [Resolute←Strong] test to expel the foulness.
III Active As above, but the creature seems to draw sustenance from the invasion into the victim’s flesh. Having pinned the character and planted their seed of rot, the victim suffers 3 Penetrating damage per turn, and the creature heals the same amount in Toughness. The victim tests for Despair on the first round, and can only expel the malignancy with a successful [Resolute←Strong] test and fire or a sharp blade – inflicting an additional one-off 3 points of Penetrating damage in the process.

Note: The rot it neither poison nor disease, but some living infestation that rends and consumes the flesh of the target. As a Lingering effect, the benefits of Steadfast apply.