Eldritch

While anima and arcana both operate (in some way) under the law of conservation of energy, there is a third energy type that acts more like a self-replicating virus - eldritch energy. It consumes both anima and arcana to fuel itself, essentially converting either of them into more eldritch energy. It is not a purposefully destructive energy, but it is prone to transmogrifying anything it touches - it is rust and nothing is perfectly stainless steel. Arcana is the most susceptible of all energy types to eldritch and breaks down most quickly.
Eldritch energy is resisted by a soul/spirit’s anima and, to a lesser but still noticeable extent, by non-soul anima. Anima is naturally more resilient against eldritch than arcana, but it’s not immune by any stretch of the imagination; it just doesn’t crumble quite as quickly as arcana. Madness is the result of eldritch’s consumption of soul anima and the corruption of the soul-signature imprint.
There are plenty of different ways to guard against eldritch energy corruption, but none of them are perfect, and none of them are long-term. Much like a virus, it adapts to what it comes up against and will eventually find a way in/through, so any protections must be multi-layered and frequently-updated.

Eldritch Souls

Some very bold, very mad magicians may try to use eldritch energy. They treat it as paint-eating lichen, letting it consume the anima of their souls without losing the original pattern. They hope that the eldritch leaves a recognizable echo of themselves so they can remain self-aware … for eldritch cannot harm or change other eldritch, and having an eldritch soul means one can use eldritch energy for magic without fear of harm. Eldritch energy (and souls) are also heartily resistant to any attacks that use arcana or anima, since the eldritch will naturally repurpose that energy for its own replication.

Demons (Eldritch Entities)

Demons are eldritch entities. This should explain everything about why they’re feared, hated, and so terribly dangerous. On the plus side, both anima and aether are good at physical healing (from illness or injury) - but eldritch has very little power over physical manifestations, making it shit for healing the body from mundane issues. So, while offensive magic is not much use against demons directly, swords and arrows and claws work great!

Interactions with Other Energies

Eldritch’s favorite snack is arcana. Eldritch can convert arcana rapidly - aether even more rapidly - and even good arcane engineering will only delay, not stop, the advance of eldritch corruption. Arcana is eldritch’s native host; arcana’s natural malleability makes it perfect for conversion to eldritch. The more eldritch involved, the faster arcana succumbs.
Eldritch can consume and convert anima with relative efficacy, although it’s less quick than consuming arcana. Eldritch tends to follow the path of least resistance in its quest to self-replicate, so it is prone to converting the anima itself without trying to change or erase the structure that the anima is in (again, this is why souls can go from anima to eldritch without losing their unique imprint). Eldritch doesn’t hold a shape/pattern nearly as well as anima does, but it can happen--it’s just noticeably less sturdy and change-resistant than the original anima.
Eldritch struggles against abyssal energy. There’s a lot of abyssal energy’s makeup that is non-reactive, meaning eldritch can’t convert it to more eldritch; eldritch vs abyssal is a very slow process where eldritch energy is forced to use the trace amounts of arcana within the abyssal in order to replicate itself and make any forward progress.

Type

Metaphysical, Demonic