Neogi

Born from the Spelljammer campaign setting, the race of aberrations known as the neogi are the most-loathed race in the cosmos. Beholders might be dicks, but they mostly try to exterminate one another rather than anyone else, and get one on his own long enough and he'll turn into a pretty chill guy. Mind flayers may eat literal brains, but they'll at least refrain from doing it in front of you, and can be bought off with slaves or other valuable things.

But the neogi have no friends, for very good reason. Looking like an ugly, halfling-sized mixer of a moray eel and a wolf-spider, the neogi mind cannot conceive of any relationship between two other forms of sapient life than owner and slave. Even aboard their own ships, there is a looooong human centipede of ownership that ends in slaves. Indeed, this is why they're less dangerous than they could be: it's really difficult for them to cooperate amongst themselves on any meaningful level without a whole lot of captains getting killed and eaten in brutal deathmatches for overlordship of multiple vessels.

Meanwhile, since they have no planet and probably blew their old one up, they jam around the universe voiping into crystal spheres to grab as many slaves as they can before voiping out. They don't even trade for them, because they can't conceive of the idea of two equal parties trading goods. The lone exception appears to be the illithids, and even they don't like hanging with the neogi.

Culturally, like mind flayers, they like palling around with umber hulks. Every adult neogi has at least one umber hulk slave, because they are slaves until they can enslave one of their own. Biologically, they reproduce every bit as horribly (and hilariously) as one would expect from such a disgusting race. Once one of them starts getting a bit senile, they fucking poison him until he's a bloated lobotomized wreck called a Great Old Master that can do nothing but eat and eat and eat, until he falls over dead and a couple-dozen newborn neogi explode out of him, eating up all his remains and then one another. How bloated? They go from Small sized to Huge sized.

Comically, this means lone neogi are almost always renegades who don't want to be turned into literal baby-factories, chased by ships full of their kinsmen out to make them do their duty to their race and make more neogi. Combat-wise, the mostly rely on their umber hulk body guards, though their bite carries a slowing poison and every one in a couple of them dabbles in magic. The Great Old Masters, meanwhile, are slow and dumb, but fire or slashing damage causes newborn neogi to explode out of them and attack their attackers. Their ships are the charmingly-named Deathspider and Mindspider, and are hard to get because everyone hates them and attacks them on sight.

They mostly got ignored once Spelljammer did, though there was one adventure where they landed on Faerun for a slaving run in 3rd edition. However, 5e eventually tossed them a bone, and they were officially added back into the game with Volo's Monstrous Compendium, which statted them out, described their life-cycle (though they now lay their eggs on the Great Old Master rather than just having him suddenly die of pregnancy), and softened them just enough that they can now cooperate with other civilizations as evil and depraved as they are in situations where self-preservation overrides their natural desire to enslave and dominate. This makes them a bit less "slave at all costs!" and more greed-driven space-slave-merchants.