One of the creepier recurring enemies in the series is the Crypt Spawn. These are essentially swarms of human brains with batwings that, ironically enough, mindlessly attack anything in their path. To make matters worse, they always appear in the presence of even greater evils, such as a timeless and bodyless... thing in the Graveyard of the Ancients, two of the Darklords themselves, and the King of the Darkness, Naar himself.
Lone Wolf has occasion to meet Naar face-to-face in one of the later books. He's described as a grotesque, bulbous, spider-like thing with a huge gaping hole where its nose and mouth out to be... and human eyes.
The Chaos-master you meet in the astral plane of Daziarn is quite the Eldritch Abomination. Its appearance is that of a vaguely humanoid giant composed of the many parts of various animals... which keeps moving and changing shape permanently.