Forms were ancient creatures that lived on Earth during the time of Silurians. They, like the Silurians, also believed in a great impending disaster and went into hiding. Forms looked like humanoid statues, with purple liquid flowing around them (like blood but on the exterior). This purple liquid is called Sultur is is extremely deadly.
Forms tried to reclaim Earth after being awaken in an old Tin Mine. The Forms arrived at a zoo where they fired deadly Sultur, destroying parts of the zoo and killing many. The Beginner stood up to the Forms. Their plan was to flood the Earth in deadly Sultur, and renaming the planet to Sultur and become the dominant species. The Beginner and Markson Frost had other plans. The Beginner messed with controls of their base and killed most of the Forms living there, flooding their chambers and then solidifying the top layer to keep the few remaining trapped there. (The Name of Earth).
When the Beginner allowed the TARDIS to pick the destination, it picked the planet Sultur. Not knowing if it was Earth or not, he landed, meeting a Form explaining that they flew from Earth and took this planet for themselves.The TARDIS was then caught in a wave of Sultur. The Beginner ran inside and flew the TARDIS to the center of the planet, removing all of the liquid into a nearby ship and leaving, realising he was defeated. (Revenge of the Forms)