First I'm going to make it clear that this is entirely my fault.
I've been GMing pathfinder and 3.5 off and on for more than a decade. But in all that time I have never dealt with characters over 10th level and usually with players that are intense with role-playing and hard casuals with character building. I was not prepared.
First my players all have god like stats. This is partly because of the system that WotW proposes called 'Focus and Foible' where you pick an 18 an 8 and roll 1d10+7 for the others. They rolled like little gods and I didn't have the balls to tell them no. So that's on me but for three of them it isn't a big deal I just kick up the power level a little.
The problem is that this Adventure Path also has an optional system for players to become vampires by investing five feats getting the abilities over time. That's a pretty huge investment and I kinda just blindly trusted that was a balanced trade. Since the actual full template didn't kick in until 9th in snuck up on me. Stats got crazy better and you got a few new tricks. What I failed to consider (was working 80+ hour weeks at the time and was pressed for time) was the defensive implications. I simply cannot threaten this dude.
Unchained Rogue 9/Anti-Paladin 2
Str 21 (15 Base, +6 vampire)
Dex 28 (18 Base, +4 Vampire, +2 Belt, +2 Leveling, +2 Human)
Con - (Undead, This is where his 8 went during ability assignment)
Int 18 (16 Base, +2 Vampire)
Wis 17 (15 Base, +2 Vampire)
Cha 24 (16 Base, +4 Vampire, +4 Headband)
AC Base 38 (10 + 3 Bracers, +3 Shield, +9 Dex, +6 Natural (Vampire), +1 Natural (Amulet) +2 Deflection, +1 Dodge, +3 Combat Expertise)
He further uses feint and a rogue trick called offensive defense (Benefit: When a rogue with this talent hits a creature with a melee attack that deals sneak attack damage, the rogue gains a +1 dodge bonus to AC for each sneak attack die rolled for one round.)
Couple with the Unchained Rogue ability Debilitating Injury he debuff the Attack bonus of those he sneak attacks by -4 (and he has feint remember)
This gives him an effective AC of 47. With Uncanny Dodge so no flanking.
Once per day he can use Smite Good (And everything scary in this campaign is good) for and additional +5 deflecting.
So on bosses that is an AC of 52.
I should further note that our longest standing house rule is that a 20 is actually a 30 and not an automatic success. Which means most things can't hit him on a 20. Why do we have this rule? No idea, it predates my DMing.
His saves are:
Fort +14 (6 Base, +7 Undead use Cha, +1 Resistance)
Reflex 26 (7 Base, +9 Dex, +7 Cha, (Anit-Paladin) +1 Resistance With Evasion)
Will +19 (6 Base, 3 Wis, +7 Cha, +2 Iron Will, +1 Resistance)
Further as a vampire he has 5 DR/Magic Silver AND Fast Heal 5.
So he is effectively immune to attrition.
As for classic vampire weaknesses the Adventure Path introduces a new feat called Blood of the Deep that removes the running water vulnerability as well as a Magic (Slotless Even) cloak that makes them able to walk in the sun. Which I gave him in place of an early treasure haul because only doing things at night was leading to constant party splitting which was driving me crazy.
Again all of this is my fault. I should have realized where this was going and nipped it in the bud at character creation.
What the hell do I do about it now though?
My players are still super exited. We have been playing this campaign for almost a year now (our longest to date) and it looks fair to continue for at least another several months. Maybe even another full year. This AP is awesome and I don't want it ruined because the encounters are trivialized. I've kicked the CR of the encounters up by about 3 in all places, increased the number of enemies dramatically and I'm using full hp pools for the monsters.
Does not remotely solve the problem, just slows things down a little.
Any advice welcome. Sorry for the text wall.