Derleth was an Elder God. He could control people emotions. The Seventh Doctor had located him in Romania in 2015. The Forge had tried to take it from the Ukranian Governement, but Lysandra Aristedes stopped the train on the Doctor's evil. When the train containing him crashed, he possessed Lysandra, and made Sally Morgan worship him. It then tried to possess the Doctor which it half succeeded. Lysandra then used the truth sayer on Derlith, which supressed him from the younger Lysandra's mind, keeping him in her mind. (AUDIO: Project: Nirvana)
Behind the scenes
Derleth is named after August Derleth, a writer who contributed to H. P. Lovecraft's Cthulhu Mythos.