Aliens in fantasy setting is indeed incredibly retarded. I'm no D&D lore expert but it's just retarded to have space ships in a fantasy setting. This 5e world seems way too bloated and could use a lot more focus.
Demons are cool tho. I wish they would lean more into the religious aspect of the world. Paladins don't really feel like holy crusaders for example. And you shouldn't be able to fuck everything that moves as a Paladin
Aside from the aforementioned Spelljammer campaign setting, released in 1989 with many supplements over the next few years, and adventure module S3 Expedition to the Barrier Peaks from 1981, there is also Supplement II: Blackmoor from original Dungeons & Dragons, published in 1975, which included the sample adventure Temple of the Frog:
The High Priest of the Temple of the Frog
Traditionally this role was voted to the most influential member of the "Inner Circle" of the brothers. It is now held by a usurper, Stephen the Rock. This fellow is not from the world of Blackmoor at all, but rather he is an intelligent humanoid from another world/dimension. Originally, he and his compatriots were sent to the area to police it against incursions of similar beings, for it was discovered that a dimensional nexus point existed in this area that allowed such possibilities. He assumed leadership of the Temple in order to have a base of power, for he now seeks glory and personal gain, with as little personal risk as possible. When the team was intact this self-seeking would have been checked, but with the death of all but the creature now posing as the commander of the guards, loyal to the High Priest and also power-oriented, most restraints are gone. Once each year the High Priest must report to a hovering satellite station, giving details of what has transpired below, and turning over any powerful "artifacts" taken during the previous time period.
This sample was later expanded upon by Dave Arneson and David Ritchie into adventure module DA2 The Temple of the Frog, published for BECMI D&D in 1986, which was followed by DA3 The City of the Gods: