…are indistinguishable from Gods.
Sufficiently Advanced Aliens are Aliens with Sufficiently Advanced Technology. They have incredible powers, which seem beyond even the powers of superscience. They may have psionics and no visible technology. Alternately, they may rely on a single all-in-one device that bends physics and serves as all the magic from technology they need in any given moment or situation (not unlike a magic wand). The important concepts are they aren't easily understood in analogs to humanity or technology - they're gods or wizards instead.
Things to do when you're sufficiently advanced:
- Ascend To A Higher Plane Of Existence
- Cultural Posturing
- Put Humanity On Trial
- Time Tourism
- Establish yourself as a god and get some of that ol' worship.
- Create life on a barren planet, and then abandon it for a few million years.
- Meddle like there's no infinite parallel tomorrows!
Types of Sufficiently Advanced Aliens:
- Ancient Astronauts
- Great Gazoo
- Eldritch Abomination
- Energy Beings
- Fourth-Dimensional Lifeform
- Mythological Characters
- Physical God
- Starfish Aliens
- Villain By Default
- Clarke's Three Laws
- Cosmic Horror
- Kardashev Scale
- Magic From Technology
- Negative Space Wedgie
- Space Opera
- Wagon Train To The Stars
Game and Story Use
- Adventure Seed: Jumpgate to Elohi
- If things are spiraling out of control, and the PCs actions are about to cause Interstellar War, it's perfectly in-genre for even more powerful precursor aliens to show up and say "We will not allow you to destroy yourselves!"
- If you want your aliens to be believable, you need to consider that they might not have human motivations. Even if starting as something vaguely humanoid and terrestrial, years of ascended powers and the distancing from the sorts of concerns we humans have may make them very odd. For example, a species that has limitless power and the ability to form or destroy worlds with a wave of the hand might not "get" how humanity fights over borders and resources. At the other end of the moral spectrum, a race that's explored the galaxy for millennia and never once encountered another intelligent race might assume we are brainless cattle to do with as they please.