Mind parasites were creatures that attached people's minds to themselves using their tendrils, trapping people in a dream world called the "hellscape", where the parasite "enjoy[ed] people's private horrors", and building "the worst thing [they] could ever imagine". When humans were released from the mind parasite's hellscape, they had side-effects of "nasty dreams" for a few weeks, but their minds would always be able to "bounce back".
One mind parasite, known as Mr Waites, fed off the Eleventh Doctor's pain in his hellscape, but the Doctor broke free and attached Waites' tendril to Waites' own mind, killing him. (COMIC: John Smith and the Common Men)