Have a party member that's into BDSM, and torture them.
But, I agree, worshipping demon lords isn't very common in most games.
Book of the Damned has some of the best boons out there, you just have to make a deal w a friendly fiend.
Nocticula let's you suck-face-ubus energy drain people or backstab for d10s
Mesmara can make you a ghost or get a grand hex
Worm guy gets you worm that walks template
Zura gives you a hideable vampirism
Sifkesh gives makes you a dismembered angel w 15ft reach
Andirifkhu turns you into a sandbox trap
Jezelda gives you true lycanthropy
Kabriri turns you into a ghoul
Mazmezz turns you into a drider
Gargoyle guy gives you a death gaze
And those are just off the top of my head.
If you do the Sifkesh boon cheese, you can get a handful of various boons that jump your sneak attack up ~7d6, not to mention A stacking flat +2/die based on the weapon. Toss that on a rogue 20 for ~200 SA damage/attack (Nocticula gives d10 SA vs. allies, just use a great disguise)
Alternately, you could use Sifkesh tricks to get your Cha mod to AC ~3-4 times