My Serpent Skull adventure path is coming to a close; I expect that it will be done in two sessions.
In the course of prepping to it, I wrote up/converted the nastiest (xp-wise) creature I have ever written, the Avatar of Ydersius, the serpentfolk god:
Avatar of Ydersius (Huge Outsider Walker 402 XP): Str 20, Dex 20, Con 16, Int 10, Wis 16, Cha 18; SZ H (4×4, Reach 3); Spd 50 ft. ground; Init IX; Atk IX; Def VII; Resilience IX; Health IX; Comp V; Skills: Acrobatics VI, Athletics IX, Intimidate X, Notice IX, Sneak VII, Spellcasting VII; Spells: Castigate II-8, Charm Person V-9, Command II-5, Counter Magic II-7, Illusionary Image VI-6, Sacred Aura-8, True Seeing-7; Qualities: Achilles heel (edged), blindsight, class ability (Assassin: black vial, Explorer: lifeline), condition immunity (baffled, frightened, paralyzed), contagion immunity, damage defiance (electrical, folk), damage immunity (acid, fire), damage reduction 10, darkvision II, everlasting, expanded spellbook (Bestow Curse-3, Consecrate-2, Fireball I-3, Freedom of Movement-4, Geas-4), fearsome, feat (All-out attack, Combat Instincts, Great Fortitude, Iron Will, Lightning Reflexes, Misdirection Basics, SC: Effect , Spell Power ), frenzy II, grappler, improved sense (smell), lumbering, menacing threat, natural defense (acid), regeneration 15, spell defense VI, superior climber VI, superior swimmer IV, swift attack II, tough III, treacherous, tricky (Shove, Venom Master), unnerving, veteran I.
Attacks/Weapons: Bite V (venomous: Virulent Weakening), Claw III × 2, Squeeze III, Noxious breath (draining, attribute III: cone 30ft range)
Treasure: 1A, 1L, 1M, 1T
Everlasting/lifeline - ydersius' avatar can only be slain by a critical hit or coup de grace with an edged or force weapon
It could have been worse, but this will kill my PCs as written, as it's nearly invincible. I am rewriting the final boss fight as a "prevent the boss from being resurrected" fight. If the skull is re-attached, it will start at -10 to all of its numerical values (skills, attack mods, DR, regeneration, etc.), but this improves by 1 per round.