Transumption

Transumption is the ritual wherein which a warlock allows Eldritch energy to consume the anima of their own soul--without losing the unique imprint of their selfhood, the pattern that contains every nuance of who they are. Even with sufficient preparations, it is a deeply dangerous process, but only transumed warlocks have absolutely nothing to fear from Eldritch infection and demonic contamination. Once one's soul is Eldritch, it cannot be harmed by other sources of Eldritch energy, no matter the volume or intensity or intent--like trying to harm a river by pouring in a cup of its own water.
Anima is difficult for Eldritch energy to transmute into more of itself, so transumption is a long process--and the most painful one a kinvar can undergo, as multiple survivors can attest. The ritual requires days of preparation for a willing warlock, and once the Eldritch has begun transmuting one's soul anima, it will take hours--up to an entire day--of debilitating physical and metaphysical agony. Merely palpating a kinvar's anima unexpectedly will usually provoke a hostile reflex, so to live through the methodical destruction of one's entire store of anima is excruciating beyond description.
With proper preparation, transumption is usually successful; the survival rate is not due to the rarity of Eldritch energy entirely losing the soul-pattern embedded in the anima. What kills those in the midst of transumption is physical shock, stress-induced organ failure, and mind-numbing madness that leads to a frenzied attempt to escape the pain in any way possible. Kinvari are not made to survive that kind of pain for that long a time with no relief, especially when the gradual loss of one's soulstuff usually indicates to a body that it is indeed dying.
Transumption is considered the highest personal sacrifice a warlock can make, and so, despite the risk and the cost, it is the highest honor one can achieve. A transumed warlock has given everything in service to their goal, be that personal power, or magical knowledge, or the ability to reduce the potential harm that unbound Eldritch energy can cause in the unwary and innocent. No warlock can be more resilient or more easily lethal than one whose soul is Eldritch.
It is possible to force transumption on an unwilling participant, but likelihoods of success and survival become very low. Most warlocks would never agree to using the ritual as a form of punishment, coersion, or forced conversion to their cause. The threat, however, has been bandied about from time to time, and to a non-warlock, it is the most horrifying thought--not only to lose one's life to Eldritch corruption, but to then survive that loss and be solely at the whim of those who destroyed whatever normalcy one may've had. Eldritch energy has been frequently compared to a viral sickness, or cancer, or rabies--and to be exposed against one's will to it is a deep-seated violation on many levels.