Tell us about Hypnos and his sons the Oneiroi, someone demands today. Geez, so pushy! But we love Greek mythology so we'll put up with it, dear questioner.
Hypnos is a child of the Protogenoi, the greatest and most loosely personified of the Greek deities; he is the lord of sleep, dreams, and oblivion, and therefore it makes sense that he is the child of Erebos, the primordial god of darkness, and Nyx, ancient goddess of night. He is the twin brother of Thanatos, the god of personified death, because to the ancient Greeks, sleep and death were two sides of the same coin, the same oblivion but with different time limits. Hypnos was less feared than Thanatos, because his "little death" was only temporary... but he also had the power to bestow and control the dreams and nightmares of sleeping humanity, which was a terror all its own.
Hypnos basically spends all of his time asleep, so he doesn't actually do much himself - he's not an active running-around type of deity, and mostly is unconscious to illustrate that he represents everyone else's collective ability to be unconscious as well. The Oneroi, however, are a very active crew, one whose job it is to fly out from Erebos every night, described as dark-winged like bats, to carry their dreams to the minds of the sleeping world. Because dreams could often contain important portents of the future in ancient Greek thought, they were more than just the bringers of pleasant fantasties or impossible nightmares; they also had the power to warn sleeping mortals of the future, encourage them to arm themselves or give them important information, or (as they did on Zeus' orders during the Trojan War when he wanted to ruin some Greeks' day) directly impart the words of the gods.
There are literally hundreds of Oneiroi, an impressive brood for a god who is usually passed out (which might be the reason that they're sometimes said to be his siblings instead of his children), and there are several of them that distinguish themselves as leaders among the throng. The most famous, thanks in no small part to Mr. Neil Gaiman, is Morpheus (literally "shaper"), god of dreams, who is the most powerful of the Oneiroi and therefore their leader, and the one who most often acts as a messenger or bringer of prophecies. His very nasty brother is Ikelos, whose name means "seeming" or "image", and who was in charge of nightmares in which visions of terrible beasts or enemies appeared, making him extremely unpopular with humanity; and to round the trio out, the third named spirit is Phantasos, whose name means "apparition", who handled all appearances of fantastic, surreal, or unreal objects and places in dreams. Only Morpheus really does very much, as the other two have jobs but not much of a mythic presence, and all three, like their thousand compatriots, are more helpers to Hypnos than important deities in their own right.
Because of his continual narcolepsy, Hypnos doesn't have a lot of myths of his exploits, but the few he does are exercises in using his power over sleep and then fleeing the consequences so he can go back to bed. Interestingly enough, he often helps his brother Thanatos head out and pick up the recently slain; he may not be about death himself, but clearly the idea was closely related enough to convince the ancient Greeks to imagine him as participating in the death scenes of heroes anyway.
In the Iliad, Hypnos appears during the crucial episode in which Hera has decided to distract Zeus to prevent him from helping the Greeks attack Troy. She knows she can can definitely get Zeus' attention with her mad sexiness, and keep it for a while, but that she won't be able to hold onto him for long enough to make sure her plan can go into effect, so she seeks Hypnos out and offers to have Hephaestos make him a golden throne if he helps her out by putting Zeus to sleep as soon as sexytimes are over. A hilarious conversation ensues in which Hypnos tells Hera he would really like to help her, but last time she asked him to put Zeus to sleep so she could get up to stuff, Zeus woke up in a rage, pursued him across the galaxy, and would probably have murdered him if he hadn't managed to get to his mother Nyx and hide in her primordial darkness, where Zeus knew better than to chase him lest he anger the ancient lady.
Luckily for Hera, she is the goddess of marriage, so she was able to up the ante by saying, "Do it and I'll give you Pasithea, goddess of meditation, who is one of the hottest goddesses in existence and also I know you've been in love with her forever, as your wife," so Hypnos is like "Okay, then, back on board with pissing Zeus off." After getting Hera to swear to hold up her end of the bargain, Hypnos did indeed succeed in knocking Zeus out to allow shenanigans to ensue, and then survived the backlash by fleeing to the forst, turning himself into a bird, and hiding in the upper branches of a pine tree whistling innocently until everything blew over.
Most of the rest of the time, Hypnos actually does Zeus' bidding, rather than acting against him (which is, after all, the smart move for a god who wants to take a lot of naps and not be barbecued by lightning). Most of his appearances involve putting people to sleep on behalf of the gods, sending the oneiroi to get things done, or occasionally helping Thanatos carry off a particularly important dead person. Regardless of his seemingly pedestrian duties, though, his powers are deceptively great; he may not often simply render the king of the gods completely unaware and impotent, but he obviously can, which makes him at least as powerful as all the ancient and primordial deities who spawned him.