Complex sentence: A gateway does indeed open and appearing from it are the Cenobites, a race of distorted creatures who practice an extreme form of sadomasochism centered around agonizing torture and mutilation; they see little difference between extreme pleasure and extreme pain.
Split into simpler sentences: A gateway does indeed open and appearing from it are the Cenobites, a race of distorted creatures who practice an extreme form of sadomasochism centred around agonising torture and mutilation. They see little difference between extreme pleasure and extreme pain.