inner dimensional sound travel
Sending stones come in pairs, with each smooth stone carved to match the other so the pairing is easily recognized. While you touch one stone, you can use an action to cast the sending spell from it. The target is the bearer of the other stone. If no creature bears the other stone, you know that fact as soon as you use the stone and don't cast the spell.
Once sending is cast through the stones, they can't be used again until the next dawn. If one of the stones in a pair is destroyed, the other one becomes nonmagical.
A pair was found in the Fissure Quasqueton city that was opened. These were created by Nithian Empire by trolls and is perpetually wreathed in darkness. The source of its power is spiritual energy.