The senchara stone is a crystal given to the party by Lord Kinekoro. The Rahjs use it to contact the party and it can also be used to release pre-loaded spells such as the one used to disrupt spirit shackles.
The party can also use the stone to contact the Rahjs, but there's only enough charge, so said Rahjs must recharge it during each call. This means that if the party annoy them, they can simply not recharge the stone and in doing so cut off all communication.