Complete Monster: Lord Mar-Vell of the Marvel Cancerverse is the evil counterpart of the heroic Captain Mar-Vell. In this universe, when Mar-Vell got cancer, he performed an arcane ritual that killed Death itself and brought the Many Angled Ones to their universe. When the Nova Corps tried to stop him from doing so, he killed all of them and proceeded to let the Many Angled Ones enslave every living being in the universe, turning them into hideous parodies. Mar-Vell's main goal is to unleash the Many Angled Ones upon many universes by killing the Avatar of Death, Thanos, destroying Death for good and leaving everything that exists as the slaves to him and his dark masters. To do this, he manipulates Adam Warlock's evil counterpart The Magus into detonating multiple inhabited worlds to open the gap between the universes and kills him when he learns Magus hasn't located the Avatar of Death. In his search, he captures a number of heroes suspected to be the Avatar of Death and tortures them for the location of Thanos when he learns that they aren't. Having left all his nobility behind, Mar-Vell stands as a shockingly monstrous villain who will torture and kill to gain what he desires and overthrow the universe's natural order.
Cosmo manages to kill a Hulk, almost effortlessly.
Thanos' plan to get himself killed, bringing Death into the Cancerverse. That is badass.
Nightmare Fuel: Every single thing about the Cancerverse and its inhabitants. Especially since some of them still act like their regular counterparts, only with something a bit "off" about them. And every time someone from the Cancerverse is "killed", they just get back up, looking even more like an Eldritch Abomination.