Rex stood his ground while the red souls of the Succubuses that he killed kept on attacking him, they claw him before going through his body inflicting another soul-excruciating pain. But despite their efforts, he just stood there.
Not one groan escape his mouth ever again, the willpower exceed the pain he got inflicted with.
It's like he has a gift for enduring whatever type of pain that the world can throw at him, even the red souls that are rigorously attacking him with excitement started to slow down. They realized that their attacks don't produce the reaction they wanted from Rex.
Just like hitting a statue, Rex didn't groan or seem to be in pain.
All he does as the hundreds of red souls keep on attacking him is look at each of the red souls passing eyes that hardly exist with a stern but calm gaze, as if he showing the red souls that no matter what they do he wouldn't succumb to the pain.