In the homebrew campaign I've made the party is coerced by a somewhat obviously named character called "the seer" into stealing a tapestry from a museum. Each character has received a letter blackmailing them to complete the heist. I figure the seer could have learned info about them via the scrying spell but I'm looking for a way that is by the rules for him to continually monitor them. One of the PC's is a spy for the seer (Though the others don't know that yet). Is there an item he could have to make reports, or maybe a gem on his sword that has a constant casting of arcane eye on it, that the seer can tune into whenever he wants. Basically I'm looking for a way that he can keep tabs beyond the spy's reports so the party has to do something more than just turn the spy to outwit him, and I'd like it to be within the rules so he just doesn't magically know every single thing they do, if that makes sense. I want him to be powerful and know a lot, but for the party to be able to do things in secret if they're clever about it. Anyway, Thanks for any advice or suggestions
I would give him an item (Ring, necklace, or even part of his armor (Bracers or helmet)) and just say it has the power of arcane eye/telepathy but you want to figure out if the item gives off any magically tell tale stuff. Does it glow, or if detect magic is cast will the other players notice it? This way the players have a chance to find out that the spy is in their midst and even lie infront of the spy giving false info. Also if you enter areas that have antimagic the item wont work and the Seer wont know what's going on. items as long as they don't over power are great tools for adventures. You could have someone trying to stop the Seer try to steal the item from the party or spy, so he could track the Seer through magic. Just make sure that the item was made by the Seer and can only be used by the Seer.
I love doing things that can lead to new adventures or allies or even enemies. Items work really well because they are portable and can be stolen or destroyed.