From publisher blurb:
A5E has been in need of a modular serpentine heritage for a while, but these aren't the tired old serpentfolk that you're used to! Sinuous trace their magical origins to an ancient alliance of nagas and couatls and have innate magical abilities in addition to the expected serpentine traits.
Originally created to serve as additional security staff around critical supernatural sites, the sinuous have since spread out throughout the world, and are a diverse (and versatile) people with multiple cultures of their own.
Included in this product are:
The all new Sinuous heritage.
Three new cultures:
Ancestral Guardian
Desert Sinuous
Swamp Sinuous
Three new Backgrounds:
Tomb Guardian
Temple Guardian
Mentor
The new Safekeeping destiny
The new Sealer archetype for the herald class
A fully-generic herald/wizard multiclass synergy feat chain with advice for using it and character concepts to get you started
The Sanctified Steel combat tradition
Three new magic items
Three sets of cultural gear, one for each new culture