Prologue : An Important Mission

Mafia!Osomatsu(boss) issue a mission to Karamatsu. It was the most dangerous mission and only Karamatsu can do it.
Osomatsu told Karamatsu to be careful and come back safe. Those were his last word to him and didn't get to see Karamatsu again until few months or years later. Karamatsu was spot. Osomatsu went over to greet him wanting to ask where have he been, however in return Karamatsu greeted him coldly,asking who is he. He's even wearing a different Mafia Outfit, a white one instead of black.
What has happen to Karamatsu during the mission?

Cover image credits: Yutif @Pixiv