Riot Spikes, an aura that gives the enchanted creature extra power, but reduced toughness. The flavor text reads "Most auramancers would have let the spikes hover just above the skin. Having the spikes rip through the skin from beneath was a touch added by Rakdos himself."
Everything Yawgmoth does. Flesh and metal were not meant to go together that way.
The freakiest thing about Phyrexia is that since Yawgmoth's death, it's actually been getting worse. (Half of this is due to improved artwork over the years. The other half comes from good old-fashioned warped imaginations.)
The original Mutilate is generally considered to be among the most horrifying card art of all time. Features include a woman's hands melding with her face and a man's neck elongating so far he begins to choke. When the card was rereleased, it was given completely new (And significantly less freakish) art.
It would likely be a shorter list to mention the cards that do NOT invoke this trope in the New Phyrexia set...
Exemplified by any non-artifact creature with the type "Horror". Just try and figure out what it was before the Phyrexians got their hands on it (or them). Then there are the ones who have types you'd recognise, but look nothing like what they're supposed to be (Good example is Blighted Agent. That thing is supposed to be Human!?)
The Simic card Rapid Hybridization lets you turn a creature into a frog lizard hybrid monster that looks like this.
Several cards in Yu-Gi-Oh! fit this trope quite nicely. For example there is the equip spell Ekibyo Drakmord, which represents a disease that weakens its host and slowly kills it, before spreading to another one. And there is also the art for the trap card Ultimate Offering, which shows a green, troll-like demon. Only his head is blood-red. And spawning from it is another, more diabolical demon, trailing what appears to be blood behind him. And the green one is still grinning...
In terms of monster cards, there is one archetype that stands out: Worms. Worms are all LIGHT-attribute Reptile monsters. All of them are hideous, unearthly abomination, but some take this just a step too far. For example, Worm Solid is a melted mass of flesh, eyes, and teeth, shoved into a glass prism barely larger than it is.
Another monster card that stands out is Parasite Paracide. Its International artwork looks normal, but its original Japanese artwork features it growing out of a mans face.