My local game group had been trying to figure out what to make of something. In this post-apocalyptic setting the world ended in the age of enlightenment about a century ago. The PC's have left their isolated Amish mountain community and ventured into an abandoned town, in there (after some other things) they plan to use it to settle the area as for other reasons it had been semi-maintained.
There was also a barricaded and partially collapsed museum. Initially routing through it, they found it intact with its exhibits still intact. Moving into the back however they found a barricade of furniture and rubble. Starting to root through the rubble they found something terrifying: A baby.
Now they knew this was in fact not a human baby, but sensing an opportunity to adhere to trope and take extra risk for style (and thus gain a tonne of awesomeness points for a shot at some re-rolls) one of the players instantly adopted the baby and took it from the museum, setting up a nursery in the quote "creepiest looking house I can find, ideally one with a rusted swing set in its yard".
Further investigation found the back rooms (past the rubble) to be haunted with ghosts of the dead. Various professors who dabbled in the black arts and had been attending to their interns rather than their study, passing off translation work on strange artifacts to new students. The hallways looked like a high powered thrasher went down the central hall, busting out of the restoration area. After assembling the fragments of a set of tablets, and deducing how they fit together (hint: its a 3d map and is a straight rip-off from "Contact", the only good thing from that awful movie..the alien looks like her dad? screw you, I want my money back".) They saw it was a ritual to summon something called a star diamond.
Time goes by and they decide after communing with some powerful magical beings to attempt to use a version of Detect Magic to see the babies true form. It was as expected, not a baby at all.
A Star Diamond is a very large (often at least bowling ball sized) crystal being composed of diamond. It has no limbs, being perfectly symmetrical. It is however orbited by a number of other smaller diamonds. This creatures has the ability to float and levitate, containing a wide stretch of seemingly telekinetic magical powers, as well as a great control of illusions and charms.
Physically it is tremendously resistant to physical damage, being a solid diamond. Its ability to lash out with its orbiting diamonds to lacerate can kill a large number of creatures very quickly.
The star diamond doesn't seem particularly violent as long as it is undetected, but it reacts with lethal efficiency when it thinks it has been exposed.
While its charm fell against most of the party with some saves, its new "parent" went full tilt to protect (though attempting to subdue other PC's as they were not under mind control) it in a bid for even more awesome points, forcing the party to subdue the character (and making their life harder), while the Star Diamond escaped.
Its utterly alien, as are its motives.