I presume its a daemon with eyes, ears, mouth, nose, etc. A swarm of insects blinding a creature and it unable to hear to determine distance; if they buzzing in his ears, if lucky could disorient him to stumble off a cliff (hopefully) or do harm to himself if attempting to claw them off, nothing to extreme, but minor scaring, enough to distract him from other actions, like killing PCs.
I dare not presume it as I uncertain, but in some games daemon do not need to breath, but a handful of insect crawling down his mouth, at least must make him feel like gagging a little?
If it could cast spells, it would need to make a saving throw to concentrate...