Dude, Ymir r@pes him. He don't need no Twilight Sword or Casket.
Ymir is equal power wise, to Surtur. And Surtur has gone toe to toe with Odin. Recently Ymir pummeled Thor who supposedly had the Odinforce (need confirmation on the Odinforce part).
Outside Mephisto's realm, it's a slaughter in Ymir's favor.
Inside Mephisto's realm, it's basically a draw. Mephisto bragged that he was the equal to an Odin or Zeus in his realm. So adding Twilight and the Casket of Ancient Winters makes it a stomp in Ymir's favor again.
Outside Mephisto's hell realm, Ymir 10/10.
Inside Mephisto's hell realm, without Twilight and without the Casket, Ymir 5/10.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
The Twilight Sword and the Casket of Ancient Winters adds Ymir too much power for Mephisto to split it.
Did Mephisto ever claim that or are you thinking of Dormammu? The only scene I can think of is where Mephisto claims his power nearly equals Odin's in his realm. Could be wrong though. The dude has a lot of guest appearances.
He stalemated Galactus. That doesn't put him above Odin in my book. Especially since every interaction I've seen with Odin points to Odin being his equal at bare minimum.
So? Hela can apparently do the same reality warping retcon that Mephisto did with Spider-Man. Those contract like agreements seem to have a lot of power -which either Breevort or Quesada commented on for what's worth- with high end Death Gods.
^ Among other things, Odin got ragestomped by Doom w/ Galactus' power.
Hela isn't retconning Spiderman. Hela may also be a Death God associating with Mephisto. That means as much as Manitou being a Skyfather associating with Odin.
Hela told moonstar that breaking of contracts after having been signed negates any future or past actions altering or reverting reality as it should be since they are bound to act within the contract past and present.
also Odin is at least Mephisto's equal and slight superior and has a slight edge over mephisto due to his divine nature..
Odin had the power to nullify Mephisto's contract over kevin's soul and invade his realm unnoticed