Marvel Comics has been using its Infinity Countdown books to build towards Infinity Wars, which we can only assume will shake up the status quo once more. So far, we know that heroes and villains alike will "reap what they sow" and that the current search for all the Infinity Stones will lead to "cosmic madness like never before" -- which makes the arrival of the new character, Requiem, all the more interesting.
The latest teaser image from Mike Deodato Jr. and Frank Martin showcases this masked entity clearly waging a war against the heroes of the Marvel Universe. The new cosmic player is depicted, with a broadsword in one hand, standing over the body of Star-Lord with a helpless Captain Marvel in the other. Impaled on the sword is what appears to be a stone-free Infinity Gauntlet, with the artwork accompanied by the phrase "DEATH WINS." With that in mind, we have a few guesses as to who Requiem just might be.
In 2017's Thanos #12 (Jeff Lemire, German Peralta and Rachelle Rosenberg), Thanos survived judgement on the mysterious world known as the God Quarry. There, he overcame his Phoenix-powered son Thane, as well as an alliance between Nebula, Starfox (Thanos' brother) and Tryco Slatterus. As part of the villain transcending his old ways, Thanos renounced his love for Death, realizing that it was beneath him to be craving someone's approval. This led to the Witches of Infinity temporarily destroying Death, after deciding to judge her themselves.
Thanos departed, leaving Nebula's team stranded. Now, Death can't really be killed, and she may looking for revenge after being spurned (reiterated in last month's Thanos #18), so it stands to reason that she could be eyeing an avatar in the field of battle. Requiem has been described by Marvel as "Death Incarnate," and by bestowing this title to Nebula, Death would also be offering the granddaughter of Thanos (Yes, in the comics, she's the Mad Titan's grandchild) a way off the God Quarry.
Conversely, the coven may have allowed Nebula to undergo the trial of judgement herself which, if successful, would grant her the power to embark on a quest to reshape the galaxy like she did back in Infinity Gauntlet.