Arcane quill

This magically infused quill has a ruby at the base of the feather. When the feather is enchanted it can communicate with another feather that is linked with it. This enchantment isn't cheap the ruby used for the feather requires at least 500 gold per ruby and than the enchantment costs for it are also no laughing matter. However when you tab the ruby of the quill a signal is being send to the other quill if the other ruby is also pressed within a minute it the one receiving it feather will float and the one that originally made the request starts to write. The other one receives the motions and copies whatever the other does. When the original stops writing the second one can write a respons to it. back and forth. To turn it off you need to hold the feather for 6 seconds to disable the feature. From the current standpoint there has been no progress to make this work through the planes though there have been several attempts. with the limited travel between the planes it has been hard to test it thoroughly.
Though this invention is now usually being used for nobles of long distance to communicate simple things. However when it was invented it was used as a way to communicate with all lines of battle and get the information across as quickly as possible. With an entire selection of feathers each for their own warcamp.

Access & Availability

it isn't a guarded military secret anymore though because of its high price it is only purchaced by nobles and maybe some rich merchants.

Complexity

The magical enchantment to make this work isn't easy and and requires a high lvl caster as well as at least a month of enchantment time depending on the proficiency of the caster