How about making it more of an equal opportunity devourer?
- Living creatures have a chance of instadeath from the aura.
- All non-hero units rise as undead after a few turns or at the end of the battle.
The risen ones that were killed by the aura automatically are under Shade's control. - Undead who enter the aura have a chance to permanently become mind-controlled.
- All undead under the control of Shade lose hit points every turn.
This may also heal Shade. - Probably doesn't need a direct "summon undead" in a battle.
- What would make sense is an automatic (random) undead summoning every turn on the strategic map. This represents "civilians" falling prey to the Shade and starting a new career as temporary snack. This would not result in a continuous army build-up because all the undead in the stack lost hit points every turn as their energy is consumed.
Shade can build an undead army quickly... even in a fight... but it won't build up a huge army because it constantly devours them, both in battle and strategically.
No real need to summon undead as well. That would be overkill, IMO. =)