Creature Enhancement – a collection of mods created by the author.

Formally, that's not entirely stupid... they are summoned using spatio-temporal technology from Shio and are a local form of life. Well, and they remain in Ashan for some time.

How to do it? A living creature is the default option unless otherwise specified (undead/elemental). In the case of demons in this situation, you can add to them 

<Item>ABILITY_ELEMENTAL</Item>

via GameMechanics\Creature\Creatures\Inferno

and at the bottom, in the ability profile for all creatures, add this line. Using the example of a regular demon, open the Demon.xdb file  and modify it like this

<Abilities>
        <Item>ABILITY_DEMONIC</Item>
        <Item>ABILITY_ELEMENTAL</Item>
        <Item>ABILITY_ENRAGED</Item>
    </Abilities>