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.
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.