In their next session, the players will encounter one of the major NPCs involved in a massive plot to free a banished god from the Abyss. This NPC (the Count Dooku of the story so far) has a way to travel from place to place very quickly, but he isn't a magic user.
The idea I have in mind, is for a portable door: A tiny trinket which, when placed in an arch or doorway, transforms into a door that leads to a location the owner of the trinket has previously been (so long as there is a suitable doorway on the other side).
There's the chance the party will obtain this trinket. So the question is, would it be overpowered and game- or campaign-breaking in their hands?
I can be talked out of this idea if there's a better way to accomplish this goal (rapid travel from place to place for a non-caster).
As requested, an in-game description of the item I'm thinking of:
Wondrous Item, (Very Rare)
This wooden carving of a door fits neatly into the palm of your hand. Twice per day, the item can be placed in an archway or doorway; it then creates a portal to a suitable doorway or matching doorway at a location you have previously visited. If there is no suitable doorway at the location you wish to travel to, the item has no effect.
The portal is large enough to allow one Large creature to pass through comfortably, and remains open as long as the user concentrates on it, up to 1 minute.
The item can be retrieved from the other side by simply reaching into the portal and retrieving it, at which point the portal closes immediately.