The Stellarians are an ancient race of non-humanoids native to a Y class brown dwarf, an ancient sphere with a history nearly as old as the Milky Way itself.(Only a million million years old themselves, the Stellarians are relative latecomers to their homeworld.) Stellaria lies deep in the Galactic Core near Sagitarius A, a region swept with the radio waves that are the most common natural food source of its only native intelligence. With a billion-year-old civilization, they have long since tamed and controlled their home's weather and native animal life - primarily other radiophiles and the things that ate them.
The natives (who in their natural state resemble ball lighting) leave their homeworld only occasionally; conditions on planets like Terra are sufficiently exotic to them (and the fleshy, organic inhabitants of worlds like that so bizarre) that it rarely seems worth the bother. They have made contact with several galactic civilizations over the millennia but they have little to offer beings so unlike them. Trade has typically been intellectual in nature - in one significant trading interaction in the Delaztri period, they traded uncounted aeons of oral histories in return for the Empire's supply of games and fiction.
Occasionally contact with outsiders has been violent (though rarely, given that their planet has no resources usable by most species) but their ability to control the electromagnetic spectrum means that such invasions have typically resulted in the rapid dissolution of the invaders. Their records are ancient (and indeed some of the oldest in the Milky Way) but you need to make them a good offer to know what they know. If your offer is inadequate, they'll take what you offer and lie to you. They have a problem with that.
One thing about being a tireless immortal with few physical needs is that occasionally one gets bored. And while their homeworld is Jupiter-sized, eventually a Stellarian will reach a time where they have "done it all". In those times, one will sometimes use their near-perfect mental control to assume the guise of a member of another species, then go live as one of those species for a time. The Stellarians do love their stories and their myths, and gaining new ones is a rare form of currency for them - a way to make the endless games of their lives a little more interesting.
Other times this trade will happen honestly - but they don't seem to quite accept the idea that other species are 'real', though - the idea that they might be hurting an organic being physically or emotionally is one that they'd have trouble taking entirely seriously...