Demons want to destroy the world. Demons are naturally malicious and cruel. They are inherently evil, existing purely to sow chaos, suffering and ultimately destruction.
Demons will throw themselves against the bars of magical cages; they make time to maim and torture any living things they find, even when that’s not the strategically correct option. Demons are inherently destructive. They’re nothing but entropy, malice and hate. Even when it’s not in their ultimate interest to destroy something, they can’t resist the urge to smash things and inflict pain.
Demons always turn on their summoners; they always turn on each other when there’s no-one else to fight, and they always bring ruin to their surroundings. Areas inhabited by demons are wrecked and irredeemably tainted. Their presence alone corrode reality like strong acid.
All demons have glowing red eyes, stink of sulfur and ash and cause holy symbols to heat up when nearby.
Fires burn more fiercely and with a weird greenish light in the presence of demons and summoned demons always crawl out of tears in reality, called Hellholes that never heal properly.
When slain, a demon crumples up and collapses in on itself, like a hollow shell getting crushed by external pressure