This sea-blue gem seems to shiver and flow, as a drop of seawater barely able to maintain its shape.

While wearing a periapt of the sullen sea, you can hold your breath for up to 12 hours before having to make Constitution checks. This is a continuous effect and requires no activation.
In addition, two times per day you can activate the periapt and sacrifice a prepared spell or spell slot of 4th level or higher to use freedom of movement (as the spell), using your own caster level or that of the periapt, whichever is higher.

Prerequisites: Craft Wondrous Item, freedom of movement, possession of a piece of the set.
Cost to Create: 3,000 gp, 240 XP, 6 days.