In many cultures there are (or at least were) certain professions or castes that were considered something other than human. Blacksmiths, midwives, ragpickers, and especially gravediggers were considered to live with one foot in another realm—sometimes the divine, more often a place of pollution and danger.
I imagine that spirit nagas are like that—creatures with one coil in another realm…dark (or even guardian) nagas who let the dark take them too far. And now they are outcasts from even naga society, let alone that of men and humanoids. Instead of guarding, ruling, or abiding, they lurk. They haunt graveyards and ruins, commune with fell powers, and otherwise exist in a world of corruption and decay, dwelling at the cusp of life and death.
In game, they’re nasty obstacles on their own. (Charming gaze plus venom plus fireball, with displacement making them hard to hit…and they get cleric spells as arcane spells, which means free healing!) But they also make great allies for the other monsters…and because of their liminal natures, need little excuse to do so. Have them round out a hag coven. Have them bolster cultists of the Great Old Ones. Have them rule over weaker reptilians like lizardfolk and serve stronger ones like dragons. When oni invade your Western RPG, I imagine spirit nagas will be the first to recognize these otherworldly monsters…and the first to throw their lots in with them. In other words, if there’s a dark plot in the works that involves powers few understand, a spirit naga could be slithering in the shadows.
I’m even (as if I haven’t suggested this before) in favor of cheating a little bit for flavor reasons. Maybe the spirit nagas in your campaign can see haunts or detect oni or shapechangers or speak with certain kinds of spirits. Nothing major or game-changing…just enough to be spoooooky on top of being evil. On a more practical, combat encounter-oriented level, maybe instead of the arcane and clerical spell list, you use the arcane and witch spell lists. Or you load your spirit naga NPC with spells you haven’t let the players use yet from a new supplement (or even from a 3.5 book like Book of Vile Darkness). After all, these creatures became outcasts for a reason. One assumes they got something quite deadly for their troubles…
A party member’s cousin is abducted into the Ethereal, but she lacks the resources to travel there herself. There is one portal to the foggy spirit plane near a certain nearby mass grave, but it is guarded by a spirit naga and her enthralled nagaji servants.
Strange masks and swords from the East have begun arriving at the trade city of Beth Tahir. The masks are becoming quite fashionable, which means they’re working according to plan…because the presence of these masks makes it easier to attract and bind spirit oni. A nest of spirit nagas is responsible. They hope to lure more oni to the city and benefit from the chaos and bloodshed such an influx will cause.
People are not dying in the town of Evenheart. The hover near death, sometimes in excruciating pain. Investigation by the Temple’s Hands reveals that the duly appointed psychopomps are not reaching Evenheart. They are being lured away and magically caged by a strange pseudo-coven comprised of a night hag, a changeling sorceress with the karmic bloodline (see Ultimate Magic), and a spirit naga attended by will-o’-wisps.
—Pathfinder Bestiary 213
My first exposure to a spirit naga was the sidekick of a Red Wizard in the pages of Red Magic, from the Forgotten Realms Harpers series. Man, 7th-grade me loved those books.
Obligatory shout-out to Serpent Kingdoms, my go-to book an all things naga- and reptile-related. I love that book (here’s how much) and urge it on everyone, system and edition be damned.