Werewolves

Werewolves get huge combat skill buffs from the current moon phase, may duel for titles, have their own culture, have their own warrior lodge, and breed faster than rabbits!

Reproduction
Werewolves are able to reproduce both normally, and by offering their "gift" to other characters; this usually leads to them being the most abundant of the races, and the hardest to get rid of. Unlike Vampires, however, a character accepting your gift does not give you the opportunity to add them to your dynasty.

Inheritance
Werewolves are probably best known for their unique inheritance mechanic, which allows any werewolf to duel his or her liege to take the liege's title. There is a law to increase or decrease the amount of time werewolves must wait after they or their liege has been challenged to a duel, but this mechanic can cause players headaches in more than one way.

Aside from the obvious risk of losing control of your realm as a Lycan due to duels, Lycans losing duels will often interrupt normal gameplay. An AI Werewolf losing a duel to a subordinate will cancel their tributary status and can end wars inconclusively due to players losing their Casus Belli against them.

Discovery
Like Vampires, Werewolf is a hidden trait until discovered. Unlike Vampires you don't take massive opinion penalties from other Werewolves after being discovered, although there is still a general opinion penalty.

Lycan Culture
While the Werewolves don't get their own religion, they do get their own culture. Founding Lycan culture can be done after being discovered and will allow a Werewolf to found a Lycan Empire.