Normal wyverns are a two-legged form of dragon, much smaller in size and not very intelligent. Slytherns, however, are possessed of a frightful cunning and intelligence, as well as the limited ability to shapeshift to a man-sized, humanoid reptillian form. They also have all the abilities of the Assassin class, at the 5th level of ability. They keep their armor class, but increase ground speed to 12.
In their wyvern form, they possess the normal wyvern abilities of flight and poisonous sting, along with the limitation of only being able to bite or use their stinging tail, but not both, in the same round.
Knowledge of Slytherns is not a commonly known fact. Only a few sages have ever even heard rumors that these mysterious creatures exist. In fact, Slytherns have killed to keep their secrets safe.
For the DM: Slytherns are a mysterious reptilian cult, much like a draconic assassin's guild, that operate in secret at the direction of their Master, an ancient Black Dragon. There is a human assassin's death cult, known as the Wyvern Brotherhood, that operates as informants, support personnel, and assassin assault teams, but the Brotherhood is unaware of the true nature of their mysterious leadership and its goals.