You can always take a clue from the helm of teleportation and roll every time they use the portals (artifacts)
If it fails they could be split across the planes.
Or
The portal could explode trapping them
Or
It could send them to a new realm, pocket dimensions etc...

That is where I would start.

I ran a playbypost rpg last year here on Steem at #steemquest
I used portals with keys that only worked one way. You could browse the old posts to see how we did it.

Hope that helps get your creative juices flowing