The Mighty Thor #400, titled "God vs. God!", is a super-sized 64-page issue published by Marvel Comics in February 1989 to commemorate the 400th issue of the Thor series. It features a main story written by Walt Simonson and art by Sal Buscema, as well as several back-up stories by various creators.

Main Story:

The issue opens with Thor facing off against his old enemy, Loki, who has manipulated the events of Ragnarok, the prophesied destruction of Asgard. Thor defeats Loki but is then challenged by another god, Seth, the Egyptian God of Death. Seth claims that Asgard's destruction is necessary to maintain cosmic balance and challenges Thor to a duel.

The battle between Thor and Seth rages across multiple realms, with both gods suffering injuries. Ultimately, Thor defeats Seth by using his enchanted hammer Mjolnir to shatter the Serpent Crown, the source of Seth's power. However, the victory comes at a cost, as Asgard is indeed destroyed, fulfilling Ragnarok's prophecy.

Back-up Stories:

The issue also features several shorter stories focusing on other characters associated with Thor, including Balder, the Warriors Three, and Beta Ray Bill. These stories provide additional context and character development, fleshing out the world of Asgard and its inhabitants.

Significance:

The Mighty Thor #400 is considered a landmark issue in the Thor series. It marked the culmination of Walt Simonson's acclaimed run on the title, which is praised for its epic storytelling, strong characterization, and innovative artwork. The issue's exploration of Norse mythology and themes of death and renewal resonated with many readers and helped to solidify Thor's place as one of Marvel's most popular characters.