oh. Ah well okay, spoilers of vol4 …
Yaldar didn’t die. He had a failed suicide attempt when Black was 11 which shocked Black to the core making him commit to stay by Yaldar’s side even if it meant he’d be beaten, which was the case for the following 7 years.
When Black remembered the suicide attempt, he only remembered the part in which he opened the door and saw Yaldar’s slit wrists and him unconscious.
By the end of volume 4, Black’s remembered everything he’s been forgetting since a certain incident. He remembered the reality of Yaldar’s suicide attempt.
Here is where Yaldar decided to commit suicide, realizing that Black will leave him messed up world. Below is them in the hospital after managing to save Yaldar’s life, and Black’s decision to stick by Yaldar.
Yaldar is alive and the last scene in vol4 shows that he’s at a sanatorium.
Hope this clears it