Ghouls and ghasts are a classic pair of flesh-devouring undead creatures dating all the way back to Advanced Dungeons & Dragons, ghasts being the more dangerous of the two. Unlike skeletons and zombies, which are created by necromancers casting animate dead, ghouls and ghasts are purported to have demonic origins (although there exists a spell usable by player characters, create undead, to produce them as well—here the lore and the rules contradict each other).
Both ghouls and ghasts are immune to poison and to being charmed or exhausted, and both have darkvision. They have high Dexterity and Strength, in that order, and can either claw or bite. The bite attack does more direct damage, but the claw attack has a greater chance to hit as well as a chance of paralyzing its target, rendering it incapable of action, movement or speech, granting advantage to all attacks against it, and turning all hits on it into crits. (Ghouls’ claw attack doesn’t have this effect on elves, but ghasts’ claw attack does.) Although its chances of success are low, its potential effect is so powerful that it has to be considered the attack of first resort, except against elf PCs. Finally, ghasts have the Turning Defiance feature, which grants advantage on saving throws against turning not just to themselves but to any ghoul within 30 feet of them. (They have resistance to necrotic damage, too, but that’s probably not going to be your players’ first choice against them.)
In my last article, I stated that undead creatures are driven less by survival than by compulsion—whatever compulsion the method of their creation imbues them with. In the case of ghouls and ghasts, this is hunger for the flesh of the living. This, combined with their average-level Wisdom, suggests that they possess a stronger self-preservation instinct than mindlessly obedient skeletons and zombies, since the whole point of eating is to fuel one’s continued existence. It also suggests that their goal is to obtain living flesh to eat, and that once they’ve achieved this goal, they have no particular reason to keep fighting.