Complex sentence: Captain Ultra's first appearance is as an applicant to the supervillain team called the Frightful Four; he is at first enthusiastically accepted by the other three villains, ecstatic at his large array of powers.
Split into simpler sentences: Captain Ultra's first appearance is as an applicant to the supervillain team called the Frightful Four. He is at first enthusiastically accepted by the other three villains, ecstatic at his large array of powers.