[Loumistro]

The Snakeskin Trait you only get if you don't kill the Naga.

There are two different ways to do that.

First of all the one I always do. Join the cult and pay every Drowrey Preist the Amount of money they ask for.

The Second get the potion use it on the Naga leave and go see Elyssia.
(Don't kill the Naga yet. Just render her Helpless.) Then talk to Elyssia and get the Trait kill her and go back and pay a visit to the Damn Naga.

Or you could do what I do and don't harm anything in the Serpent temple except spiders and monkeys.