Let's not veer away from the way undead generally work. Either create spawn if killed in any way by the spirit (like most MM undead), or via energy drain only (like a vampire). I'd prefer the former.
Regardless of the case, I'd prefer "all or nothing" on the enslavement...either the spawn are free-willed, or they're enslaved.
In other words, "Two wrongs don't make a wright."
Neglected spirits can assume wraithform at will
It looks like I stuck the wraithform ability in the wrong thread!
Wraithform (Su): A wraithworm may become incorporeal for up to 10 minutes per hour. It may not use its natural attacks or paralyzing gaze while incorporeal.
Skills: 5 at 11 ranks
Wights have Hide, Listen, Move Silently, Spot. Add Knowledge (history) since they are ancestors?
Feats: 3
Multiattack? Power Attack? Reckless Offensive?
Environment: Any?
Organization: Solitary? (Add wights if we decide upon enslavement)
Challenge Rating: 5? (A wight improved to 8 HD is CR 4, and these add Con damage and unholy toughness)
Treasure: None (like wights?)
Alignment: Always chaotic evil
Advancement: 9-16 HD (Medium)? (This follows the wight's "double HD, same size category" advancement)