We play the anniversary edition. We are just before the confrontation with Karzoug! Pcs just got into 20th level with a lot of good weapons and artifacts. The short story is that the Pc's after dealing with the goblins and clear the Thistletop took that place and build a castle (home) for them and their loved ones during the campaign. At the end of 5th chapter, and after they have deal with the Skinsaw Men, Forever man planned his revenge, and when they came back to their home everyone where murdered and only one letter awaiting for them from The Forever Man.
So yes i want him to be powerfull enough and sneaky. So Inquisitor may be a nice option. He has to be at least 23 CR or else he ll be an easy target for them i think