Cain performed a solemn nod as he indicated for the Collapsing Lightning Duke to stand up. The man had proved his bravery and loyalty multiple times over the last year, and it was through his protection that he survived the Blood Sea's Civil War, so Cain trusted him with some of his secrets.
Of course, the Collapsing Lightning Duke was not the only one who had gained Cain's trust, but he was one of the few who could keep the secret. After all, even if there were those like Levi or Zarak who would rather die than betray Cain, that did not mean things were so simple if they fell into the hands of an enemy.
Cain was sure the Soul King would not have wanted to betray the Void King since that man granted him the power to take revenge on the Umbra Clan. Yet, Cain still obtained access to all the memories of the Dark King and used them to fight against The Organization.