Marquis of Death

The Marquis of Death is a supremely powerful supervillain from Marvel comics and is considered by many as one of the most powerful entities to ever be created by Marvel comics - this is a bold statement in a multiverse populated by innumerable reality-warpers and cosmic-entities but is largely supported by the events that took place in the saga that involved this infamous entity. The Marquis of Death's involvement in current Marvel lore is unknown but as of his defeat in the original saga he is not deceased but rather imprisoned in a mysterious room - it took the combined efforts of almost all Marvel's heroes and villains and an alternate version of himself to weaken the entity sufficiently for Doctor Doom (himself greatly amplied) to strike the final blow and end the Marquis of Death's reign of terror upon the multiverse as a whole.

Personality
the Marquis of Death is a deeply cruel and wicked entity driven by a desire to "punish" others, yet in reality he is simply a sadist who enjoys mass-murder and torture - the definition of a "mad god", while it is extremely common for reality-warpers to gain this mindset due to the omnipotent nature of their powers the Marquis was exceptional in his cruelty and made a game out of mental, physical and spiritual torture of heroes and villains alike, warping reality in horrific ways and offering false deals to bait heroes into impossible odds (for example demanding Reed to allow his own son, Franklin, to die in order to "spare" the rest of reality).

Abilities
the Marquis of Death is a reality-warper of unlimited design, his potential ranks far greater than what we have currently seen in Franklin Richards and may even exceed those of Mad Jim Jaspers - this is shown by the fact he could create cosmic entities so powerful they made existing Marvel entities (such as Death and Galactus) seem like "insects" in comparison and with a simply wave of his hands he could obliterate entire armies of villains and heroes, he could even cause a chain reaction among stars that sent every star in a cosmos supernovae at once, killing said universe in a near-instant.

Marquis could hurl Doctor Doom to the prehistoric era with a simple portal, mentally command Galactus and even stronger entities with no apparent strain - he could theoretically accomplish absolutely anything he desired without limit : he would, of course, be significantly below the level of Marvel's "One Above All" (who is a monotheistic God and thus completely beyond all power-scaling) : it is also probable that he was below the level of the Living Tribunal, yet it is unknown for sure as neither fought or even interacted with one another.

The current level of power for the Marquis in the rebooted Marvel multiverse is unknown but presumably just as vast, though he is able to be contained via unknown means by a door created by Doctor Doom - even this required him to be severely weakened by the combined efforts of a large majority of Marvel's heroes and villains, an alternate version of himself and Doctor Doom (who had been empowered by millions of years of magic).