I have tried numerous times now, but the Bishop no longer provide the charm oil. You get somekind of automoton potion that reduce aggressiveness, but not the actual charm oil needed to open the portal. That is after trying to kill to get it and after trying the seed ritual (5 or so times). The ritual gives you the potion and killing just gives you snapdragon oil. This is from the desert gauntlet and all other realms. (the ritual was peformed in the desert gauntlet realm, over and over.
Went to Swamp. Found Bishop with the "red" skirt (not just the regular ones) and yes it was by the portal in the Swamp. Offered the ritual garden. Got Automoton's potion. I am getting really frustrated with such basic problems in this game.
Yes, it must be a in the Swamp Hunt realm now not a desert. When you pop out of the portal onto a loadstar you are facing this giant robot statue - the fabled automaton bishop arena is inside! Just fly across to it and enter to make your offering.
It needs to be a Fabled one
If it helps, the Bishop you need is in the Swamp Gauntlet, not desert. It's in a huge arena, and NPC's have a habit of picking them up before you can.