There is a demon summoning mechanic in the game. Of course, since you play as the demon, you won't be the one doing the summoning. Instead, you'll be the one getting summoned:



Throughout the game, you'll come across several summoning circles with varying requirements (level, forms or weapons equipped, etc.). If you meet those requirements, you'll be able to answer the call of whichever foolish human has decided to risk his soul in exchange for your services.

(Summoned demons are akin to psychic projections, so even if they can interact in a limited capacity with the living world, they remain trapped in Hell)