Great setup, looks like it could be fun!
Bone devils are smart and the devil has time to plan and to strategize together with the evil priest. Stare hard at its list of SLAs and powers. Right away it should try to summon a second bone devil. (Called creatures can do this.) If you roll it straight and it makes the 35% chance, bam -- this becomes a much more challenging encounter.
If you don't (or don't want to), fair enough -- the devil has lots of other things it can do. Wall of Ice means it can wall the PCs inside the temple! "Vwoosh." "Hey, what was that sound?" "Vwoosh... something just covered up the windows..." Then Teleport+Invisibility + Wall of Ice *at will* means it can slip in, throw up some walls between PCs, isolate a single PC, go after that PC with a full round attack, and then use quickened invisibility to disappear again at the end of the round. Lather, rinse, repeat -- it can do that up to three times.
Or Major Image 3x/day. It casts an image of a scary monster and gets the PCs to waste resources attacking it. After a couple of times the PCs catch on? That's fine -- now Evil Priest, who has quietly slipped into a side alcove, casts Summon Monster IV or V. Whoops, this time the monster WAS real! Dang! (And if the PCs are abusing Invisibility, make it a creature with Blindsense or Scent.) Then the bone devil casts Major Image again. Then Evil Priest casts Spiritual Ally, causing an appropriate fiendish creature (a "servant of his god") to appear. The Ally can attack, doing d10+3 damage, but it can't be hurt except by Dispel Magic and a few high level spells. Then have him throw Spiritual Weapon into the mix -- an invisible blade wielded by nobody. Your PCs should be starting to freak out a little at this point.
-- Suggestion: establish in advance that Father Joe is a nervous type who is very worried about facing an actual DEMON FROM THE ABYSS. Have him constantly running in and out of the main temple area to splash holy water around, burn incense, move holy icons in and out... "This icon of Saint Bluto should repel the forces of darkness... oh wait... it's ancient and fragile, I really can't endanger it..." Then he has an excuse for being out of the room for a round or two.
Also, do have Father Joe cast a few real, no kidding help and buff spells on the PCs. But just a few. Players are usually pretty good at reverse engineering an NPC's level, and you want them thinking the priest is much lower level than he really is, so throw a Bless and a Magic Vestment in advance and then have him run around doing the occasional Cure Light Wounds.
Meanwhile, did you notice that Bone Devils are immune to fire? Because PCs usually aren't. Have the devil wall them inside the temple with Walls of Ice and then set the whole place on fire. Evil Priest of course knows this is coming and pre-casts Resist Energy on himself (Fire resistance 30, which means he can completely ignore an ordinary fire.) You can mention early on that the temple is made of wood, or at least has a wooden roof and lots of wooden pews, flammable tapestries, etc. Let the PCs smash their way out... and then have the devil cast Wall of Ice again, requiring a Reflex save for PCs below a certain initiative to avoid getting trapped inside.
("Won't the fire melt the Walls of Ice?" "You remember that 'vwoosh' sound you heard when the walls started appearing? Well, now you're hearing it again, but more faintly. A second row of walls is appearing, just behind the first. You're being double-bagged...")
Finally, just before the devil appears, it meets in a side room with Father Bad Guy... who hits it with a bunch of clerical buffs. Bull's Strength and Cat's Grace for combat, Owl's Wisdom for Will saves, Bless for +1 attack, Shield of Faith (+3 AC), Entropic Shield (ranged attacks miss 20%)... and then have him cast Shield Other! This will give the devil another +1 to attack and to AC (and at this point it's like +4 to attacks, +6 to AC -- your melee PCs may find this encounter a bit frustrating). But if they do hit the devil, he only takes half damage -- half the damage gets transferred to Evil Priest. Who starts crying out that something invisible is attacking him! (Best give him a few ranks of Bluff, just in case. But given invisible foes, illusions, Spiritual Weapon, and Spiritual Ally, this is really pretty plausible.) Then Father Joe has an excuse to cure himself instead of using cures on the PCs. (Heck, maybe have him fall down and play injured. Then the PCs can waste resources curing *him*.)
-- Want to save Father Joe for later? (1) Set the place on fire and have him "die" in the wreckage. (He is of course fireproof, and has the dead body of some unfortunate homeless person stashed downstairs to provide a burned corpse.) (2) Have him cut off by a Wall of Ice -- he dashed into a side room to get an icon, or maybe was overcome by cowardice. Boom, Wall of Ice traps him away from the PCs. Then they hear screaming, horrible screaming...
Honestly, if you play this cleverly, a bone devil plus an 11th level cleric -- even one who's in disguise -- could be a very deadly encounter indeed.
Have fun!
Doug M.
