The Hoothi are one of the most disgusting monsters in the entire Whoniverse. A race of Festering FungusPuppeteer Parasites, they can take over any organic body, either living or dead and instantly make them explode into a hideous shambling mass of mushrooms and rotting flesh. Even their spaceships are pure nightmare fuel; gas dirigibles the size of a moon, stitched together from the putrefying skins of millions of their victims, and inflated by the gas all that decaying meat gives off. It only goes From Bad to Worse when they arrive at a planet that's basically oneenormousgraveyard...
During the skirmish with the Aubertides, the student who's manning the Vickers gun turns around for a second to talk to John Smith, gets hit in the back of the head with a dart, mutters "I'm sorry", and then his head explodes. It splatters blood and brain matter all over John Smith, Bernice Summerfield, Joan Redfern, and the room full of school children sitting next to him. Smith then clings to the body in shock, rocking back and forth while the neck stump still spurts blood.
The fates of the people who tried to push through the time barrier.
The fate of Gallifrey should the Aubertides get the Doctor: They breed like crazy and take the Time Lord Citadel through sheer force of numbers. We see Flavia and Romana on their knees before them, whilst August asks them for a link to the Matrix. When they refuse to give it to him, he slices open Flavia's neck so she'll regenerate and they can keep on chopping. He then turns to Romana and we are left to assume she will suffer the same fate. Oh yeah, and the members of the High Council are being paraded around, hanging from poles.
Russell T. Davies' contribution to the novel series, features a cavalcade of horrors including an undead half-mechanical drug dealer, machines exploding out of people's heads, a man experiencing hellish hallucinations while delirious from being stabbed (and giving himself makeshift stitches with a sewing kit), and a woman killing her husband by lovingly preparing a rat-poison-laced dinner and forcing him to eat it in front of her.
'These candledays you can only see up the West chimney,' Jobiska said sadly. 'Cousin Luton thought he could climb up the East chimney, but he got stuck. We could hear him regenerating for eleven candledays. That was five hundred and six years ago and he's still there.'