Complex sentence: Allen the Alien is an Image Comics character who first appeared in the Invincible series as an apparent alien aggressor, who Invincible's father, Omni-Man had fought several times in the past, all without talking with him.
Split into simpler sentences: Allen the Alien is an Image Comics character who first appeared in the Invincible series. He was an apparent alien aggressor, who Invincible's father, Omni-Man had fought several times in the past, all without talking with him.