Mark had tied one end of the chain to the Cerberus' leg, but the other end was inside the Styx, and the lost souls in there were pulling on it relentlessly!
BARK! BARK! BARK!
Mark stood up calmly as he cracked his neck to one side while ignoring the intense barking and seething from the Cerberus. He knew that it was a gamble to carry out a plan like this. Mark did not know if it would work. The River Styx was an unfamiliar place to Mark, and Mark did not know if the souls of the undead would bother grabbing hold of inanimate objects. But Mark knew that if they did grab hold of it, it would definitely be the end of the fight. Mark and Charon were unable to fight against the souls of the undead when they were pulling him down into the River Styx when he died the first time. That meant they had more than enough strength to drown Cerberus!
Drag!
BARK! BARK!
Thanatos is that guy.
P.S. Thanatos was really impressed by Mark's performance, but he was also pissed at the fact that Mark did not die. Gods tend to express emotions based on their domain, so Thanatos expressed his hatred for Mark far more than his content at Mark's performance.