I really thought we were going to lose the Xill in the OGL debacle - and we kind of did. They do have a replacement in Monster Core called the zeuci! They are considerably more bug like and implant larva into their eggs rather than eggs, but they clearly fill the same niche. In some ways, this makes the zeuci even worse than their predecessors. Instead of immobile eggs, the first stage of a zeuci infestation involves the larva wiggling around under the host skin. It doesn’t do anything mechanically, but worms under your skin is a special kind of disturbing body horror. If your table is comfortable with it, play it up for all it’s worth.
In the Tian Xia World Guide, Zecui feature prominently in the Valashmai Jungle, in 1e they were Xill bound to the Universe. Unfortunately, the ORC transition did drop one of the cooler bits about the Xill - their ties to the Ethereal Plane. This makes sense from a legal distinctiveness perspective, but it is a bummer for me. Nothing is stopping you from adding that tie back in at your own table, however! It can be a call out to the game’s publishing history, even if it can’t feature in products any more. Paizo won’t send Pinkertons after you!
An occultist coven has recently unearthed ancient Thassilonian research into the zecui and taken it to Xin-Shalast. With this knowledge, they believe they can not only survive a Zecui infestation, but bend the larva to their will. The rapidly growing cult preachs that it will usher in a glorious age of human-alien hybrids. Based on their trials so far, they are more likely to leave a trail of twisted bodies and madness in their wake.
Zecui who worship Xhamen-Dor appear to be immune to the Star Seed’s corruptive influence. These nomands implant spores from the Inmost Blot along with their own larva. If one infection does not consume their victims, the other one will, birthing either an alien fanatic or an undead monstrosity. The few who survive are still cursed, dreaming forevermore of lost Carcosa and developing mindscapes linked to that foul city.
While those who know of zecui can think of few things more nightmarish than their larval infestations, such a thing does exist. Xekk is brood speaker, who can telepathically control larva and guide their development. Even worse, she has a taste for torture and worships the kyton demagogue Morrobahn. Like her patron, she "helps" improve her victims with grafts and surgery, inevitably mediated by her psychically commanded brood. Those captured by the sadistic zeuci experience infection not once, but again and again.