Harald Jaekelsson and his Undead Viking band has discovered a portal leading to different worlds, he then proceeded to invade and possibly conquer these different realities
World 1 : The Breaker/New Waves
World 2 : Ghost in the Shell verse
World 3 : Black Joke Verse
World 4 : Baki-verse
World 5 : Luther Strode Verse
World 6 : History's Strongest Disciple Kenichi
World 7 : Air Gear Verse
World 8 : Tenjo Tenge Verse
World 9 : Hunter X Hunter Verse
World 10 : One Piece Verse
So, for context, Harald Jaekelsson had been cursed by village shaman/wise man a thousand years ago and he had killed him as he did so. This meant that the curse was left to run unchecked and became a thousand times more powerful than it was meant to be. This meant that Harald was, for some reason, completely immune to damage and could somehow damage even Thor who, in question, hit Harald on the head with Mjolnir full out and his wrists broke.
The curse the wise man lays upon Jaekelsson and his men turns them into unstoppable engines of destruction capable of killing anything on its path and nobody, except those who share blood with the man that cursed them in the first place, can even harm (let alone kill them).