Sending is a long-distance communication spell: a message of up to twenty-five words, cast through the air to a recipient who can answer in kind. The reply must arrive within one to five minutes. Sender and recipient must share the same plane of existence; outside it, the message falls into the void.
The guardians of Ardis Vala communicate through a “sending” (spell or artifact, the record does not say) under the same twenty-five-word limit. An unreliable source adds that the spell risks being overheard by third parties.