Jormungand
The Midgard Serpent is a major player in the prophecy of Ragnarok. Jormungand leads the side of evil in the Evil vs. Good fight. Thor will manage to kill the serpent, but it will cost him his life. It will also be such a clash that Asgard itself will be destroyed in the process. Asgard won't be going alone though...all of creation will be killed as well.
Thor #294- The newly created Odin creates enemies and gods to be the all-father of. The Midgard Serpent is sent spiraling off with the other foes.
Thor #273- flashback-
To prove that his strength is greater than
Jormungund’s to his fellows gods Thor rows out to see on the
boat of Hymir the giant. Thor uses the head of the biggest ox as
bait. He throws his line in and the Midgard Serpent bites it.
Hymir says that he was afraid of this, but Thor says that he was
counting on it. Hymir says that Thor is a fool to try and cheat
the Norn Fates. He grabs Thor’s line and cuts it with a
knife. The Midgard Serpent is the one who got away.
Thor #278- As they learned by Volla’s prophecy as Loki battles Thor (albeit in an altered manner than that which was foretold) the bridge they are on collapses. The Midgard Serpent appears. Thor hurls the sword that he uses as a replacement for Mjolnir, who is now in the hands of Red Norvell. The sword bounces right off Jormungund’s face. Thor lifts a huge rock and hurls it at the Midgard Serpent. Jormungund blasts the rock with eye beams that melt it. Thor goes to grab another rock, but Jormungund strikes Thor unconscious with its tail. The Warrior’s Three rush to Thor’s aid. Volstagg says he will sit on the serpent’s tail. They are thrown about as the Midgard Serpent turns to fight the arriving Red Norvell and Sif. Norvell stands on the serpent and starts smashing it with Mjolnir. Asgard is being destroyed by Jormungund’s thrashing. Thor notes that Red has the strength to handle Mjolnir, but not the skill. Red throws the hammer back to Thor. Thor flies at the serpent, but Jormungund retreats to the depths beneath Asgard with the mortal Red. Thor peers down the hole, but suddenly finds himself outside Asgard. Jormungund surfaces near Thor. Thor sees that Red is not with the serpent and assumes that he is dead. Thor says that he will avenge Red. Thor strikes Jormungund over and over. Jormungund keeps getting up until Thor hits him with one more immense blow. Jormungund vanishes in defeat. Odin later reveals that he set up illusions and circumstances to make it seem as if Ragnarok was happening, but that the fight never truly reached Asgard.
Thor #327
Thor #379
Thor #380-
Thor
flies over the body of Jormungund as it encircles the Earth.
Jormungund tells Thor to give it up and that he cannot escape
him. Jormungund says that he knows of Hela’s curse on Thor
that he is unable to die that nullifies Volla’s old
prophecies. Jormungund says that after he has reduced Thor’s
skeleton to dust he will break open Thor’s armor and suck
out the marrow. Then Thor will live forever in the belly of the
Midgard Serpent. Thor wallops Jormungund with Mjolnir.
Jormungund’s face hits the ground. The earth groans under
Jormungund’s weight. The serpent swings itself into Thor,
which sends Thor smashing into a hill. Jormungund says that if
Thor were smart he could have avoided this fight for centuries,
but Thor keeps playing the hero after Hela’s curse. He says
that he also knows of the legend that they are to kill each
other, but seems odd that Jormungund is in prime form while Thor
wears armor to protect his brittle body. Jormungund says that he
will erect cairns to Thor using buildings of Midgard, and that
the World Trade Center is too small a monument for Thor. Thor
throws Mjolnir into Jormungund. The serpent screams in agony.
Thor flies up to continue the assault, but Jormungund bites and
traps Thor within its mouth. Thor smashes out Jormungund’s
teeth and escapes. Thor calls upon the storms and lightning. He
says he will become one with Mjolnir. Jormungund charges Thor
with his Death Thrust. They collide. Jormungund falls over. Thor
drops to earth ablaze. His armor hits and is now hollow.
Thor #486- Malekith has tricked Kurse into letting him out of Hel. Kurse defeats Hela and takes over Hel. Kurse then heads for the surface and sees Thor. Malekith’s magic causes Kurse to see Thor as Malekith. As the new ruler of Hel Kurse brings forth Jormungand, Fafnir, and a number of other dead enemies of Thor.
Thor #487- 
Kurse orders his servants to attack. The Midgard Serpent
swims through the ocean destroying ships as he goes. After Thor
re-kills Nidhogg he spots Jormungand creating great waves in the
ocean. Thor flies down to stop him. Thor is only distracted for a
second as he notices that the Godpack have arrived to help Sif
battle the others, but it is enough as the Midgard Serpent
quickly swallows Thor. Thor starts pounding and pounding on
Jormungand’s stomach, but the poisonous gas within his
stomach weaken each of Thor’s successive blows. Thor loses
unconsciousness. Beta Ray Bill and other Asgardian warriors
arrive to help fight.
Thor #488- The Midgard Serpent chews on the harbor tugboat as Thor passes out inside its stomach. Beta Ray Bill rips open the Midgard Serpent’s stomach to find Thor. Bill climbs inside to try and get Thor up. While inside the hole Bill made reseals. Bill also finds that the Serpent is harder to hurt on the inside. Thor and Bill strike together. They break a hole and fly out. The Midgard Serpent disappears back to Hela’s realm.
Avengers vol. 3 #1- The Midgard Serpent is seen in the South Pacific fighting a group of Avengers lead by the Black Widow, and including Swordsman, Firestar, Sersi, Magdalene, and Rage. The fight ends when Morgan LeFey uses Scarlet Witch's magic to enable her to lift the Twilight Sword enabling her to alter reality.
Thor #127- flashforward- As the oceans swell it signals the arrival of the ultimate destroyer- the Midgard Serpent. The Midgard Serpent rises up before Thor signaling Ragnarok and the death of the gods.
Thor #128- flashforward- In the prophecy by Volla the Prophetess, the Midgard Serpent leads evil in the ultimate battle between good and evil. The battle causes Asgard to be destroyed. The explosion effects everything killing all.
Thor #273- flashforward- Again Jormungund is seen terrorizing Asgard in the prophecy of Volla.
Thor #200- flashforward- Volla the Prophetess prophecies
that Loki will lead an assault on Asgard triggering Ragnarok. As
Thor and Loki battle the Midgard Serpent appears. Loki goes down,
but Thor blasts the ultimate destroyer with power from Mjolnir.
The Midgard Serpent hits Thor knocking him unconscious. Balder
and Fandral attack Midgard Serpent, but their swords shatter
against its skin. Hogun wants to try his mace, but the Midgard
Serpent turns to attack Loki who is firing upon him with his
mightiest weapon. The Serpent destroys Loki’s weapon without
a thought. Thor wakes up. He flies onto the Midgard Serpent and
strikes it with all his might. Thor manages to kill the Midgard
Serpent, but Asgard is still “rent asunder” and the end
will come.
Thor vol. 2 #80