At level 20, the warlock becomes infused with Infernal/Abyssal/Fey Power. The resistances gained from the warlock class become immunities. Choose another 2 energy types: these become Resistance (10). The warlock becomes an outsider(evil) or outsider(chaotic).
The warlock(lvl 20) can use his Fiendish Resilience a number of times each day equal to 1+ his Cha Mod.
To Paizo the class, I give it d8 hit die and 4+int skill points with the following skills.
Acrobatics, Bluff, Craft, Disguise, Intimidate, Knowledge: Arcana/Planes/Religion, Profession, Sense Motive, Spellcraft, Use Magic Device
