Summon Demon + Sheep sounds good.
I first thought that fire rain / fireball could be added, but as it does not really work on dragons - especially on BDs -> the demon summon sounds much more alluring (using them as tank/distract unit) + sheep - works great on lower leveled but outnumbered units (like 1800 archers
)
Actually what I would add : No retaliation ability (but maybe I am too nostalgic about HOMM 3)
Or instead Sheep - some BUFF spell could be added
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GIFT (Demon army is strong on their skills - but they may run out of these skills and for longer battles the demon army is weaker and weaker - so free GIFT would be nice)
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mass Initiative... (well - this may sound overpowered, but when Undead have so many speed improving options - Rina, Undead Commander, why not demons?)
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Magic Spring ? ... Would this be too much ? (In combination with summon demon...)
Or - instead of skills, give him some more resistances - so he could be really used TANK unit - and his teleportation ability would be more of a benefit (I would not send archdemon in enemies frontline - as god armor does not work on him and stoneskin + his natural restance = 50% DMG resistance... not to mention that many enemies have magic/poison attacks...) -> Would just GOD armor work on him
I personally would create new ability:
GAZE of DOOM -> would be like alchemysts acid srpay - but astral damage - with his teleportation ability this could be greatly used as "hit and run" strategy, possibly hitting the most dangerous units no matter how good they hide
- so his resistances would not be important anymore - using Wait + Teleport this would make him THE SOLUTION for destroying enemies strategy, ignoring their TANKS and using their "grouping" agains them