In the middle of the moon forest near Akakura there is a circle of small stones. It is very hard to find and only the eladrin hunters seem to know about it. The key is a cut of the passenger’s palm using a silver dagger (one point of damage).
This portal goes both ways.
In the fortress of Perhalkux Dram within the Stigmaris Mountains on Avernus, the dark lord Ghoroton maintains his production of weapons and pools evil sorcery to nefarious ends. Having an unguarded portal lead from Celestia to Avernus and back can only mean one thing: Powers in high places want this portal to remain as it is.