Complex sentence: Calling himself the Composite Superman, Meach left the message in both heroes' bases to meet him on a mountain and told the heroes that if he was not allowed to join their team, he would expose their secret identities, which he learned via telepathy.
Split into simpler sentences: Calling himself the Composite Superman, Meach left a message in both heroes' bases to meet him on a mountain. There, he told the heroes that if he was not allowed to join their team, he would expose their secret identities, which he learned via telepathy.