Complex sentence: The original X-Men encounter a group called the "Sons of the Vanir" who summon Ymir to Earth, and after a brief battle Thor uses his mystic hammer Mjolnir to return the character to Niffelheim.
Split into simpler sentences: The original X-Men encounter a group called the "Sons of the Vanir" who summon Ymir to Earth. After a brief battle Thor uses his mystic hammer Mjolnir to return Ymir to Niffelheim.