A chasme is a demon added in SLASH'EM. They are unusually fast for a demon and possess a sleep gaze attack that is very dangerous if you do not have sleep resistance. Fortunately they are only randomly generated in Gehennom, by which time you will almost certainly have acquired sleep resistance.
A hostile chasme is extremely dangerous if encountered before you have sleep resistance. Blinding yourself with a towel or blindfold will negate the sleep attack, but poses it's own problems if you are not telepathic. Being invisible also gives a chance for the gaze to not connect (The chasme gazes around, but misses you!). Reflection will also protect you, and can put the attacker to sleep if you are not invisible.
Chasmes make superb polymorph forms as they fly, move very quickly, can put monsters to sleep, are inediate, and are capable of demon gating (but incapable of wearing body armor!). The gaze attack can be used manually with the #monster extended command, but is also done automatically when attacking in melee. Using the gaze attack in melee on a reflecting monster will cause it to be reflected back to you - chasmes are not sleep resistant, and therefore it is possible to put yourself to sleep if you have polymorphed into a chasme before acquiring sleep resistance. Also, like all gaze attacks, it fails if you are invisible yourself. Attacking bare-handed also gives a chance of summoning a tame chasme. A tame chasme is a good pet due to their speed and ability to put monsters to sleep, but chasmes are traitorous and have a chance of spontaneously rebelling. It is not advisible to demon gate a chasme until after acquiring sleep resistance.