The God of Thunder, Thor, has one true nemesis: Malekith, the Dark Elf Sorcerer King who has inflicted pain and suffering on Asgard for ages, and is now back after his crushing defeat and exile at the end of the War of the Realms event. Malekith was imprisoned in Hel, forced to withstand the worst possible torture for eternity. Thanks to the deadly symbiote known as Carnage, the Dark Elf walks free again, and he is ready to inflict his revenge on Asgard.
Malekith is one of Thor's classic villains, who also appeared in the MCU movie Thor: The Dark World. He is a Dark Elf born in Svartalfheim, one of the Ten Realms of Asgardian cosmology, a dark, cruel land whose inhabitants grow up to be ruthless killers who have to survive in that harsh environment, ravaged by constant wars. Malekith's life was also scarred by war, but he believes that conflict is needed to forge the souls of the Dark Elves and make them stronger, so his purpose in life is to spread war and destruction across the Realms. Malekith accomplished it during the War of the Realms, when he used the Mangog to destroy Asgard and conquered every world except Midgard (Earth), where Thor finally defeated him after a long and gruesome battle. Malekith was killed and his soul was sent to Hel (the Asgardian hell), where he was crucified and forced to watch his innocent child-self play with puppies for all eternity, the worst possible torture for the cruel Dark Elf.
However, not even the gods of Asgard could predict that the Carnage symbiote's quest for ultimate power would bring him to Hel. After being separated from its former host, the serial killer Cletus Kasady, Carnage set into motion a mysterious plan that will lead him to "ascend" and become a god. He has stolen the powers of other supervillains and used them to travel to Svartalfheim first, where he has possessed one of the Mythical Hellhounds sacred to the Dark Elves. His next stop is Hel, where Carnage needs to kill Malekith to complete his ascension to godhood. However, Detective Jonathan Shayde, who is now hosting Cletus Kasady's consciousness in his body, is trying to stop Carnage. In Carnage #6, by Ram V, Rogê Antônio, and Erick Arciniega, Shayde gets to the crucified Malekith first but, rather than killing him (which would stop Carnage's plan forever), he decides to set him free, and the two will try to escape Hel together.
It is hard to predict what Malekith's freedom will entail. During War of the Realms and the very long Thor storylines that preceded the event, writer Jason Aaron managed to establish Malekith as one of the God of Thunder's most effective and convincing villains. Maybe is too soon for him to make a full comeback, however, if Malekith manages to escape from Hel (and form Carnage's murderous plans), his next step would certainly be to start planning his revenge, not just on Thor and Asgard, but on the entire Ten Realms. A lot changed since Malekith's death: Odin was killed, Thor is now the All-Father, and Valhalla was destroyed in mysterious circumstances, all things that Malekith could exploit as potential weaknesses in Asgard's armor.
The former Dark Elf King, however, will first have to evade the deadly Carnage. It's still unclear how exactly the red symbiote plans to "evolve" and what role Malekith's murder will play in this. What is certain is that Thor and the entire Asgard will have to sleep with one eye open, now that Malekith the Accursed is once again on the loose.