Stonewall Runes

As an action, once per long rest, you may touch
the two stones together for 5 seconds, and then
separate them. A wall or floor of solid stone up
to 10 feet square by 1 foot thick appears between
them for 1 hour. (AC 17, 40 hit points)
Often used to craft temporary support structures, these
earth-powered runestones are seldom found outside the
secluded caves of the stone giants.