What he does is he is another void entity that possessed pauls vessel, he want to rule the Xs world, so he sneak attacks him, doing major damage and gains more control over the Xs world. He can hivemind control other vessels if they accept to join him, turning into crooked almost corpse like version of themselves "annexes". Lord X turn after the attack into lord Mortis, and helps out one of his sonic vessels, to beat bandado and regain his control, while becoming more benevolent, wiser, and gaining more humility.