More info about using the Grimoire Viperian (available here:
http://rpg.drivethrustuff.com/product/104112/Grimoire-Viperian) material to supplement the Serpent's Skull adventure path:
The book has a great deal of snake-themed material ideally suited for the adventure path, and a ton of generic-themed material that can be dropped into any campaign.
Just some of the snake-themed material from the book:
Shapeshifter base class:A new base class focused on shapeshifting abilities such as alter self, shapechange (similar to a druid's wildshape - so many snake forms are available), forms of the mythical beast, dragon form, giant form, special form powers, favored form ability enhancements, and more.
The class is perfect for many of the villains in the adventure path, especially the serpentfolk. It also makes a great class for characters.
Monsters:New
deadly venomous snake types with much more powerful poison and realistic poison side effects. Drop these into nearly any Serpent's Skull encounter, especially the wilderness sandbox areas.
These are much deadlier than their standard Bestiary counterparts and will rightly put new fear of serpents in your players
and make combat with them exciting and memorable. There are also suggested rules for using these deadly snakes in summoning spells and with druid shapechanging abilities.
Black Mamba
Krait
Gaboon Viper
King Cobra
Spitting Cobra
Rattlesnake
dire version of all the above
Other new snake-themed monsters include...serpentine medusas
scaly otyugh
trog ettin
trog troll
xocouatl (evil necromantic couatls)
Monster templates such as...half-serpents
sentient serpents (with levels or sorcerer or other classes - these make great minions for serpentfolk bosses, or can even be introduced as new sub-bosses - they're the last thing most players will expect!)
vampiric serpents
amphisbaena serpents
hydran serpents
serpentine undead
snake-vomiting undead
viperian dragons
Supernatural Signs of Evil templates such as...Sign of the Vile Serpent (apply this template to any of the serpentfolk or their minions to create unique foes with unexpected and cool serpent-themed powers).
Sign of Death
Sign of Evil, and others
Prestige classes:Serpent Warlock, Serpentine Necromancer, Serpent Temple Warrior, Serpent Druid, Silent Adder, Python Master, Cobra Master, Snake Clan Warrior, Snake Cult Leader, and Viper Assassin.
Any of these can be used to spice up the Serpentfolk adversaries and their minions, or allow your players to pick them for their own characters.
Spells:Serpent Strike, Amphisbaena Call, Speed of the Mamba, Summon Snake Swarm, Summon Venomous Snake Swarm, Serpent's Wrath, Virulent Venom, Ghost Serpent, Serpent Reflexes, Serpent Sneak Attack and Serpentine Whip, and Wall of Serpents.
These spells are great for a serpent-flavored campaign, and many are designed to go along with the prestige classes and other new material in the book.
Eldritch Path Feats:Secrets of the Beast and The Secrets of Terror.
Magic Items:Cobra Scale Armor, Mamba Scale Armor, Viper Scale Armor, Serpent Scale Cloak, Python Spear, Serpent Strike Weapons, Snake Flail, Strangling Snake Whip, Weapon of Deadly Venom, Snake Animal Totem Headdress, Banner of the Supernatural Sign.
The book includes a ton of additional material from all these categories that is not specifically snake-themed.