Blood and souls for my lord Arioch really isn't what I have in mind (though Michael Moorcock is phenomenal

).
Here's what I ended up going with:
Tear of Night: The
tear of night is a mithral
spear +10 that inflicts an additional 1d8 points of divine cold damage per strike, or 5d8 on successful critical hit.
The tear of night grants the Greater Critical (spear) and Improved Critical Multiplier (spear) feats any wielder who meets the prerequisites.
If the
tear of night delivers the killing blow to a opponent the wielder can imprison the soul of said foe in the
tear of night with a successful level check (DC 11 + targeted creature’s HD + divine bonus). The
tear of night can contain up to 8 souls at any one time, with immortals imprisoned count as their divine rank in prisoners. A non-divine soul can be used to cast a single non-epic spell at no cost, including no XP cost so long as the XP cost is less than or equal 100 * the level of the soul. A divine soul can be used to cast a single epic spell at no cost (including XP costs and backlash damage).
Pricing
+14 +10 enhancement bonus (10 * 1.4)
+6 divine cold damage (as divine ability: reduce energy by 1 step to 1d4, reduce overall damage by 1/2 to multiply on crit)
+6 soul stealing as divinie ability
+2 grant 2 feats
=+28