Serpent Men appeared twice in my campaign as of now. Both times in dungeons. I didn’t exactly planned it like this at first, but now I think I like it that way. Serpent Men, for my personal use, are dungeons/tombs guardians, adding to the more typical undead ones. And that’s also why I won’t call them Yuan ti, the official D&D (licensed) name, as they won’t have much in common, apart from the appearance and statblock. Indeed, in the canon lore, the Yuan ti have been for a long time portrayed as your They Hide Among Us infiltration-type and behind the scene manipulators. Very much cliché by now and not palatable to me…
Aside : A few years ago I’ve read The Face in the Abyss from Abraham Merrit (of appendix N fame). In it there’s a place called Yu-Atlanchi ruled by the Snake Mother. Well, if you squint a little you can find right there the origin of Yuan ti, the name of the D&D serpent men. How they’re portrayed come from other sources, but enough digression.
Immortality at a cost
One thing I’ll keep from the lore is that Snake Men are transformed humans and not an evolved species (even if I prefer the latter most of the time). Snakes, in real-world mythology (not your watered-down D&D mythology drek), have long been associated with immortality (with their seemingly magical skin-shedding ability) and that seems a good place to start. The ritual to transform them in their new form also gives them prolonged or maybe even eternal life. As for the cost… Well, the cost is looking like a friggin’ snake the rest of life isn’t it? That and slithering around a dungeon for ages, waiting for intruders, ugh!
Sssuscetible to Sssuggestion (snake-charming)
Part of the ritual, and their acquired snakelike nature, is their susceptibility to suggestion (not them doing the suggestion!). That’s a highly desirable trait to those powerful nobles/sorcerers and such who wish to have faithful servants to guard their tomb/ palace/dungeon. Nearly immortal and loyal to boot: a perfect minion! Of course, wily intruders with acces to enchantment/charm magic can absolutely abuse this weakness and greatly diminish the threat that Snake Men pose.
Harking back to their first apperance in D&D in the tournament module The Dwellers of the Forbidden City by David Cook, Snake Men come in different flavors, from the almost human to the snake-with-arms type (the so-called Abominations).
Since then a few more flavors have appeared, culminating so to speak in the Yuan ti Anathema:
This “ultimate” form is attained (in my version) by the fusing together of 3 to 6 individual Snake Men in a painful but quick ritual, if an emergency requires it (i.e invaders in the dungeon).