Iblis are demons formed out of roiling black smoke, with vaguely the same shape and size of a human figure. In the midst of the vapor is a pair of burning yellow eyes. Iblis demons tend to attack in groups and surround their prey, though they are not very bright.
In Idris in 2007, a bunch of Iblis demons cornered Magnus Bane in an alley in Cistern Square. The Iblis had formed a line in Magnus' path, forcing him to retreat back against a wall. Magnus managed to hit several of them with spears of blue fire, but others just took the place of those that vanished. One of them, however, seemingly more cunning than the others, drifted around Magnus and reformed behind him. It was about to strike when Alec Lightwood snuck up behind it and killed it. Magnus dispatched the remaining Iblis demons, leaving the pair alone.
After a brief conversation about their relationship, a dozen more Iblis demons flooded into the square, already fanning out into a half circle around them. Alec promised Magnus that he will introduce him to his family if they lived through the battle, which they did.