Complex sentence: The birth of Karthikeya is narrated in mythologies thus: that an asura named Suraabaathman--a sincere devotee of Shiva--performed severe penance to an extent that Lord Shiva was pleased by his austerity and appeared before him.
Split into simpler sentences: The birth of Karthikeya is narrated in mythologies thus, that an asura named Suraabaathman a sincere devotee of Shiva. He performed severe penance to an extent that Lord Shiva pleased by his austerity and appeared before him.