Complex sentence: After assembling the Young X-Men in the Danger Cave, he gives them their costumes which have the appearance of the standard yellow and black training uniforms and lectures them that he is reforming the X-Men beginning with perhaps the last generation of mutants.
Split into simpler sentences: After assembling the Young X-Men in the Danger Cave, he gives them their costumes, which have the appearance of the standard yellow and black training uniforms. He lectures them that he is reforming the X-Men, beginning with perhaps the last generation of mutants.