From Tardis Wiki, the free Doctor Who reference

The Lobri were creatures that were created by humanity's unconsciousness that fed on humanity's fear and hatred. They existed in another dimension; a plane of existence where humanity's collective unconsciousness resided. From at least 1932 to 2082, they were clients of the Threshold who were hired to find a way out of the dimension they inhabited to allow them to manifest themselves in same dimension that humanity physically inhabited. (COMIC: Ground Zero)


The Threshold attempted to use regular human beings, transferring them to the plane on which the Lobri existed to allow them to feed on raw human fear, to prepare them for the transition to normal reality. But the regular humans were unable to cope with the transference. So instead they selected companions of the Doctor who had experience with inter-dimensional travel. (COMIC: Ground Zero)

Peri Brown was taken from 1938, Ace was taken from 2082 and Sarah Jane Smith was taken from 2086. Susan Foreman was also taken from 1963 as the Threshold mistook her for a human. (COMIC: The Curse of the Scarab, Black Destiny, Operation Proteus, Ground Zero) Peri Brown, Sarah Jane Smith and Ace were transferred to the Lobris' plane to sustain them in preparation for their transition to the normal plane of existence. During their 'feeding' process, Ace was able to destroy most of the Lobri using Nitro-9 seemingly fatally injuring herself in the process. However the one of the Lobri was able to escape to Earth. They were prevented from feeding and distrupting humanity's collective unconsciousness by the Seventh Doctor, who disabled his TARDIS' safety interlocks, allowing Susan Foreman to materialise the TARDIS within the Lobri, killing it. (COMIC: Ground Zero)