First, as a reminder:

There are five mana mixes that can be done, since the GMs said that Life and Holy can't mix, as far as making mixed mana spells.

So, the 5 "legal" mixes are:
Sorceries:
Elemental & Unknown (Blackfire)
Holy & Lunar (Feral Magic)
Holy & Elemental (Domination)
Lunar & Elemental (Evocation)
Necromancies
Life & Lunar (Perversion)
Life & Elemental (Corruption)

Blackfire, given the written histories of the Warrior Mage guild has to be Elemental mana, and may or may not include one or more other types.

Holy & Life as a combination does not exist, which is why Feral Magic must be Holy & Lunar, since the other two sorceries are defined, and the only Necromancies are Perversion & Corruption.

Now, if Clerics are barred from Necromancies, that sounded like a decision that was made by the Cleric Guru, so that is between them and their guru.

Every spell caster using a multi-mana spell, or even a spell from another mana type than their own, is at risk of a potentially dangerous backfire. I would think that a multi-mana spell is more dangerous to backfire than a single mana spell in general, with the following ranking of difficulty/danger from most dangerous to least dangerous:
1) Multi-mana spell using both mana types different than your own
2) Multi-mana spell using your own mana type plus another mana type
3) Single-mana spell from another mana type
4) Single-mana spell using same mana type from another guild
5) Single-mana spell from your own guild not available from your guild leaders
6) Single-mana spell normally available for your own guild

So, underpowering a type 1 spell can cause it to take both your arms off, while underpowering a type 6 spell just leaves you with a collapsed matrix.

Overpowering a type 1 spell may cause similar problems, I have not heard of anyone reporting using too much mana, just problems from the spell not getting through their target's MR, so this is speculation; while overpowering a type 6 spell just causes backfiring, with nerve damage as a possible effect.

The mind behind Kynevon

My guess is that Feral magic is Lunar, Elemental, AND Holy. That would definately fit the name "Feral" - powerful as hell but just as likely to annihilate the caster. That's just speculation as of now since there are no Feral spells, but I think it's a safe bet.

Kalyn