Basically, it would allow the player to capture souls of enemies, and use them to create enchantments and minions.
A rough idea for the skills would be:
Soulstealer
At first level, allows the player to hold 1 souls. Each level adds another soul that can be held at one time.
These would be inventory items (listed as Soul - "Creature" "Level"). Naturally, stronger souls (higher levels, or bigger creatures, like dragons) would be worth more.
Souls can't be Transmogrified or sold (unless you put a soul merchant in somewhere maybe). If dropped, they just disappear permanently.
I think Soulstealer should be a free sustained ability. Basically, while it's on, if you kill something, you get its soul.
If you're at your limit of souls, you wouldn't get anything, but should probably get a message telling you you're "full".
Soulcrafter
Allows you to imbue a soul into an item. This would work sort of like with gems. However the effects would be FAR more powerful.
Because is it stands, gems into items give you rather crap items compared to most random stuff you find. This skill would let you make things on par (or stronger if you find the right souls) with items you would randomly find as you play.
Also, you can keep adding souls into an item. However the drawback is, the more souls you put into something, the more chance of it exploding in your face. Maybe 5% per soul.
Higher levels of this gives bonuses to the quality of items that you make. As well as reducing the explosion chance of items (maybe down to 1% per soul at level 5 skill).
So say a fire dragon soul adds +30 Fire Damage to a weapon, having higher skill in this might let it add say a 10% bonus to per skill level. So you'd end up with +33 at level 2, +36 at level 3, etc. Maybe try to balance it out so that you get a nice round number at level 5 (so you get +50% at level 5... or maybe 100% bonus).
Soulfeast
You devour a soul you're carrying to get an effect.
This effect would be different depending on the type of soul. Orcs might get you an attack buff. Dragons could get you a resistance buff to whatever type is appropriate (fire for fire dragons). Maybe even class specific ones, so a mage soul might give you a mana or mana regen buff.
High levels could give you a % bonus to the effect.
Soulmaster
You can breathe new life into a soul. You use this skill, and select a soul you're carrying, to create a minion from that soul. Basically summoning whatever you'd killed in the first place to fight for you.
This minion is a temporary one. Maybe lasting 50 turns or so. Enough to hopefully turn a battle, but not to have an uber army following you around.
More levels in this skill could extend the duration of how long your minion lasts.
Possibly even giving a bonus in stats to your minion.
Thoughts?