Complex sentence: The autistic young mutant known as Dummy became a student of the Xavier Institute before joining when his powers manifested he became a sentient gas put in a latex suit that grants him a humanoid appearance and functionality and later takes part in its Special Class.
Split into simpler sentences: The autistic young mutant known as Dummy became a student of the Xavier Institute before his powers manifested. When they did he became a sentient gas, which was put in a latex suit that grants him a humanoid appearance and functionality and later takes part in its Special Class.