The Questline for Revendreth outright states that it’s Red Anima is acquired by Souls letting go of their Sins…
The funny thing is that the Black Sin Elementals aren’t generated from Anima but Black Tar-like Substance(though still produce Sin Anima for combat)! I suspect that Sin Anima is Anima created by Souls letting go of their Sins and is not the actual Sins themselves!
Case in point: the Souls sent to the Maw are Souls stripped of Anima by the Venthyr… If the Soul lacks Anima yet retains actual Sin when cast into the Maw wouldn’t that mean that Sin is not the Red Anima but the Black Tar-like substance?
Considering the Maw Death Elemental Mount is literally called Sin-Touched Deathwalker shouldn’t that mean that the Death Energies brought by the Souls filled with Anima into the Maw are being mixed with the Black Tar-like Sin Energies from the irredeemable which have no Anima?