WARNING: The following contains spoilers for "Reign of the Swamp King" from Dark Nights Death Metal: The Last 52 -- War of the Multiverse #1 by Justin Jordan, Mike Henderson, Adriano Lucas, and Dave Sharpe, on sale now
Thanks to his elemental connection to the Green, Swamp Thing is typically a very powerful DC hero. The Justice League Dark member spent much of Death Metal hindered after his power was diminished due to the machinations of the Batman Who Laughs. However, he also absorbed all of the lingering Green into himself. In the final stretch of the conflict, he got his full strength back and more -- but for a steep price.
In Death Metal: The Last 52- War of the Multiverse, Swamp Thing just defeated his Dark Multiverse counterpart and may have gained his ultimate upgrade in the process, but at the cost of some other notable "monstrous" DC characters sacrificing themselves.
With the last heroes and villains of the DC multiverse fighting for their very existence, many of them have been confronted by their darkest forms across Death Metal. This includes Swamp Thing, who finds himself confronting the Swamp King. In the Dark Multiverse, the Swamp King had once been very similar to Swamp Thing. But without Alec Holland's spirit, just his memories, the Swamp King's rage was far harder to control. Becoming obsessed with "killing death," Swamp King absorbed Arcane, which gave him control of the Rot and the Avatars of the Red, and he subsuming their elemental power for himself. He grew so powerful that even the Parliament of Trees could not stand before him. This led Swamp King to reach his ultimate form and become powerful and undying -- and the only living thing on his world.
Luckily, Swamp Thing's connection to Alec Holland's spirit gives him the ultimate weapon against the Swamp King. If he can reach the monstrous version of himself, Holland's soul can overtake the Swamp King and essentially give Swamp Thing control of his form, forcing the Swamp King to cease being just the monster and allowing Holland to take over. But to reach the Swamp King, the plan requires that a handful of powerful figures hold the line against Swamp King's forces. While none of them are pure Avatars of the elemental forces, each is connected enough to them to present a distracting challenge to the Swamp King: Frankenstein (the Red), Clayface (the Melt), Solomon Grundy (the Green), Brimstone (the energies of the Dark Multiverse), and Arcane (the Rot).
Each of them fights the Swamp King's forces, and do their best against his armies. But ultimately, all of them are overwhelmed and killed in-turn. Their mission is a success, however, and Swamp Thing reaches Swamp King long enough to overtake him. Unable to destroy himself to kill Swamp Thing -- as it would mean the end of all remaining life -- it seems that the Swamp King is essentially defeated and Swamp Thing takes his connection to all living things left in the world. The story ends with the physical remains of the Swamp King falling to the ground amongst the bodies of Swamp Thing's fallen allies, while the heroic Swamp Thing stands triumphant atop his new kingdom.
While this is definitely a victory for the heroes -- keeping the Swamp King from interfering heavily in the conflict at the heart of Death Metal --, it also comes at the cost of both heroes and villains. It could also lead to Swamp Thing having a major power-upgrade post-Death Metal, which could explain how his Future State incarnation has been teased to become so powerful. Although he's already a massively strong hero, this transformation could be the key to making Swamp Thing one of DC's most powerful forces.