I like the idea with a play on life shards though. I might build on that. Like resoln with death shards.
Yeah... the angels could have some sort of blessings that buff them in combat. And the more life shards you have, the more powerful they become. So they're the most magically connected faction. That means they scale with magic, and become very powerful in the end-game. Unit cost = roughly the same as others. But strength? Weak in start, but when you have 5 life shards they'll be strong as mustard, and you're not paying any more for them.
There's lots of interesting interactions we can make with shards. For example, we can make a super-unit in the unit designer that actually drains "shard power" from the faction while alive. With such a concept, you'd be able to design super-units like Archangels or Dominions.
I'd have to see the AI use the "corruption" spell though. Otherwise it is no good.
Instead of making them just life-shard dependant, different shards could be granting them different bonuses.
Each life shard: +10% health and +10% life healed at start of every tactical combat, +2 magic resistance
Each fire shard: +10% attack and ignores 10% of armor
each earth shard: +10% defense, ignores 10% of all physical damage
each water shard: 20% chance to ignore prone attacks, +1 accuracy
each air shard: +1dodge, +1 move in tactical combat
It actually fits fairly well with the story I have for them. They were agents of some god before the cataclysm, but in the eyes of that god the angels failed to safeguard the world. As a result, they were cast-out from the heavens, and lost the ability to fly. But now they find this new power source, and it makes them go zoom-zoom. (ps. they remain fanatics, as they believe they can work their way back into the good graces of the god. their change from agents to a full-fledged faction is because they are taking a more direct approach since their previous method failed so hard)
edit:
They would also have a unique building like the living stone, it can be built once. It would give 1 life shard power, and decrease unrest in local city by 10%. In addition, they gain diplomatic bonuses (not related to the improvement)
Yeah, I'm feeling quite good about this concept for angels. It defines them as the shard-powered faction (not necessarily the "magic" faction). They would be late-game bloomers.