Plot
editDuring the winter festival, Dominarian artificer Mishra Planis is devoured by an awakened sea titan, Marit Lage. Devastated soldier Kytheon Akros, who has fought anew for two years as the planeswalker Gideon Jura, is called to Dominaria alongside pyromancer Chandra Nalaar, much to the disdain of the Dominarian Guild of Mages. Marit Lage has left a message for Gideon at Mishra's icy peak. The following night, Marit Lage devours the veteran planeswalker Teferi and leaves another message for Gideon. Gideon and Chandra discover that Marit Lage left an ice capsule in Mishra's vault containing evidence of Mishra's claiming by a Phyrexian, Gix, on Mirrodin—a technocratic plane ruled by Elesh "Atraxa" Norn,[a] who is vampire bloodlord Sorin Markov's creation. While Atraxa pleads ignorance, Gideon notices that Gix's blood partner Hera Vess is living on the plane as a sorceress. When Gix disappears, Gideon sends Vess to Mirrodin for answers and discovers that Teferi, Mishra, and the mage boggart Slobad are in debt to Sorin's house.
Marit Lage submerges Slobad, makes him a Phyrexian iron artificer, and sends him to interrupt Mishra's state funeral. When Gideon arrives, Marit Lage signals him via Slobad's planar powers and threatens to crush him if Slobad cannot assimilate three sea artifacts. Slobad refuses to touch the third—the congealed aristocratic prison realm of a vengeful Guild of Mages reject, which was used only once to end their corrupted angel Avacyn's killing spree—and is killed. Gideon and Chandra deduce that its creator may be Atraxa and track her to a blood feast. They discover that Avacyn's destructive mission was passed on to Sorin, with many dark mages involved. Vess inadvertently exposes them when she arrives to barter the revival of her father and discovers Gix as an assimilated artifact in a small iceberg. Gideon captures Atraxa after a battle of minds, but he and Chandra learn that Atraxa is not the creator.
Gideon and Chandra follow Marit Lage's passage to the ruins of a besieged sea race initially supported by Akros and Theros, where they learn that Marit Lage holds a grudge against the Akros knight estate for failing the campaign. The naturalist and planeswalker Nissa Revane goes into hiding after intercepting a structural cage meant for Kytheon. Marit Lage sends whispering evidence to the surface that House Akros, which was conquering Theros against internal and external enemies before its defeat, hired Sorin to kill Avacyn's acolyte who was preaching the truth about Theros' extermination of its sea devils. Kytheon confronts Nissa, who maintains that Akros only hired Sorin to threaten the acolyte into silence; upon learning that Sorin had murdered the acolyte instead, Akros planned to give Sorin back to the guilds. Nissa believes that Sorin brought down Akros and Theros to save himself.
Vess reveals to Gideon that Sorin is her father. After learning that Sorin captured Gix for discovering that he was the creator, Vess tries to kill him, but Gideon and Chandra arrive in time to stop her. Chandra captures Sorin, but Marit Lage kills Sorin in a directed engulfing wave moments later. Marit Lage is unmasked as sea wizard Arenson Tenebris and dealt with by the Golgari conclave, where she tells Gideon she took inspiration from him when targeting the evil and destructive. Gideon learns that Tenebris has trained the Kjeldorian priests by emanating a planar presence for thousands of years that will culminate in the death of general Tsabo Tavoc. The priests suicidally conjure a tsunami from Dominaria to Ravnica which flood both planes. Tenebris' followers attempt to kill Tavoc, but Gideon and Vess manage to stop them. In the aftermath, Tenebris befriends another underground prisoner of deep Golgari magic,[b] while Vess deems Dominaria beyond saving and heads for Innistrad, in chaos since Sorin's death. Gideon aids recovery efforts and vows to inspire hope in the multiverse.