A nightgaunt is yet another Lovecraftian monster to appear in SLASH'EM. Small groups of them are commonly seen in later levels of both the Dungeons of Doom and Gehennom. They possess a unique attack, AD_TCKL, which behaves exactly the same as a paralysis attack except for having a different message, and abusing constitution in addition to dexterity.
The paralysis attack is no particular threat, as it is affected by magic cancellation, and one is highly unlikely to encounter these monsters before having MC 3. They are mostly a threat for being hard to kill; having a base level of 15, even with your best weapon they will take quite a few hits. Since they are generated in groups and are quite fast, like the bats with whom they share a glyph, they can be quite a nuisance. The amount of physical damage they do is not particularly high, but from two or three of them over the period of time it will take to destroy them, can wear you down. Note that they are capable of picking up and using wands and other magic items.
Chatting with a nightgaunt will produce the message "The nightgaunt squeaks.", likely a holdover from bats. This is an interesting image, given their Lovecraftian origins.