Derleth

Derleth was an Elder God whose domain was jealousy, having the ability to control people's emotions and make them become obsessed with worshipping and protecting it. Darleth's followers would often become so overcome with jealousy that they would eventually turn upon and kill each other.

The Seventh Doctor had located him in Romania in 2015. The Forge had tried to take the Truthsayer from the Ukranian government, but Lysandra Aristedes stopped the train on the Doctor's orders. When the train containing him crashed, he possessed Lysandra, and made Sally Morgan worship it. Itthen tried to possess the Doctor and half succeeded before Lysandra used the truth sayer on him, suppressing him from the younger Lysandra's mind, keeping him in her mind. (AUDIO: Project: Nirvana)

Behind the scenes
Derleth is named after, a writer who contributed to H. P. Lovecraft's Cthulhu Mythos.