Will of The Mark
This is a magic item used to convey a message to a pre-determined target
Sending Stones allow messages to be transmitted between themselves over large distances on a single plane. The stones must be keyed to one another; two randomly-selected sending stones will likely not be compatible, though with the careful application of a sending spell cal be keyed to the same frequency. When two stones are synced in this manner, they lose their connections to their previously-connected stones.
Unfortunately, these stones tend to be found by themselves, as they are rarely kept together (this would undermine their usefulness). However, a locate object spell can help locate the destination stone.
Price: 5,000 gp. Create Wondrous Item; sending.