@ataris You need to place that item in shield slot to work. And you didnt mentioned what hero class you play (warrior, paladin, mage?) At start every battle it will give you option to place Asmodeo unit on arena, but it must be placed near some of your units. It`s beholder type units with 3 active skills (but mystic ritual feel free to disregard since it`s almost useless). Every action with that unit give him individual exp that isn`t connected to hero or pet dragon, and with using him to attack, or when he retaliate he gets exp. If your new to Asmodeo try him out, but there is 2 type to his playthrough : 1) use only him to kill enemy so hell lvlup faster, and when he have enough hp and retaliations jump him to enemy lines and watch show
2) is only use him as support, which is half-ass strategy but still interesting.
In start try to concentrate on his hp -Titan , hp regen -Accelerated Metabolism, lifesteal -Vampirism, and retaliation -Boisterousness, act again after retaliation -Unstoppable skills lvlup.
Also important are : Carapase, Elemental mastery, Speed, Elan (bonus to initiative) Sixth sense (miss chance)
Skills to avoid : Necromancer (gives more charges to mystic ritual, useless mostly), Destructive beam, Beholder commander, Creepy, Domination, Harvest. I`m not saying this skills are useless, but try to lvlup first skills that buff Asmodeo before giving him skills that debuff enemy, because he first need to stay alive to use them. Also try not to use lightning attack (blink in enemy units) early in game while he is weak. but if he dies you can revive him with couple magic crystals. Since he is magic immune try leading couple units with him to get guardian medal it will help him stay alive later in game (i say somewhere on forum that players got him up to 60lvl when he had 8000hp and other sick stats, but that were mostly solo Asmodeo play).
But as i said before try him out, and have fun