Sacred clay pouches (class 5) are made by using class 5 sacred clay on a Creation kiln. They require 80 Summoning to make and wear. They can be used to summon a level 139 class 5 Clay being that can store up to 24 items.
When you die any familiar you had summoned will disappear and will drop any items they were holding on the ground where they were standing when you died. Many players, particularly mages, though theoretically anyone could use this tactic, use familiars to their advantage in this situation. When the familiar disappears and drops the items it is holding the items, for a short time the items are only visible to you (similar to if you were to drop the items yourself). For this reason many mages use familiars to hold extra runes so that if they die they can run back, pick up the runes, and resume fighting without having to make more.