Han Mengli arrived at the Shadow Manifold realm, a world that might've been his home had his parents survived. They were born in Bloodhaven, home to the Blood God, but had joined under the Dark God, so they would no doubt have come here to join the cult.
Thinking he could maybe find his father that way, Han Mengli chose to join the Dark God's cult, although technically he had never left it to begin with. After arriving at one of the cult's branches and explaining, he was told that he could join after a few simple tests.
Those people were already accepting more members to help them with the war, so rejoining would be as simple as showing a few skills to prove that he could be of help.
His friend, the blood-haired young man, decided to join the cult along with him, now choosing to forego his previous name and call himself Steelheart.
"Brother Han, you should give yourself a daoist name too," Steelheart said as they waited for the registration.