The entry for lillendi is big and juicy, full of obvious hooks and interesting quirky details. I think the difference between them and, say, the last entry, is that they're intelligent. Being able to assign desires and motivations and unique social behaviors really goes a long way towards being able to use a "monster" for something other than "the monster is menacing innocents!" or "somebody hires you to kill one for various reasons!".

But, yes. Lillendi. Talented artists (to the point of having the various and sundry abilities of a 7th level bard), defenders of the wilderness, servants to the gods of the moon, etc, etc. They come in both male and female (true to D&D's nature, the only artwork I've ever seen for them has been female, so I was actually a bit surprised at the male bit) but, interestingly, reproduce via parthenogenesis: even those who appear, act, and identify as male are, technically speaking, biologically female.

There's a few other cool bits. A lillendi can choose the hour of her or his death, called the Silent Hour, during which they fight with a "calm, focused fury" (automatically winning initiative, +4 to attack rolls, double damage), and at the end of the hour (or earlier if they die during it) they dissipate into mist, as well as a combined moonbeam/chaos spell. If they die by accident without having entered the Silent Hour, it's viewed as a horrible tragedy by other lillendi, as this death means their essence is unjoined with the power they serve.

There's also a bit about lillendi social status: it's determined by "a simple system of initiations into mysteries and the ownership of certain totem masks". So, basically, a culture of secret societies: something I can totally get behind. Masks, music, magic, mysteries, and the moon. A lot to work with.

* Legend says that the lillendi long ago served the Powers of Law, but abandoned said service, and now serve the gods of the moon instead. A sage will pay the PCs handsomely if they can discover why.
* A group of settlers seek to build a new home in Ysgard. Unfortunately, the local lillendi, who consider the area their territory but haven't marked it very well, discover the settlement after the settlers have set up their new homes. It's a bit too late for them to move on and select a new place, and the conflict is on the cusp of violence when the PCs stumble upon it.
* A lillend is tricked by a Baatezu into fearing for her life enough to enter the Silent Hour prematurely. She implores the first people she stumbles across (the PCs, obviously) to deliver her mask, flute, and scimitar to her sisters and brothers of her secret society (which, naturally, will have a lot of questions about how the PCs got such effects, if the PCs can even find them.) And getting some revenge on the devil in question would be nice as well, if the PCs can manage it.