Web Novel/Timeline

Birth of a Demon Lord Arc
By offering the corpses of the Blood Shadows and the 15000 human soldiers, Rimuru summoned an Archdemon and his two greater demon subordinates. Using the corpses they were successfully able to materialize. He ordered them to capture the the three escaping Blood Shadows, and bring them to Ranga.The secret revival art was lacking the necessary energy. so the Archdemon offered his two subordinates. They were delighted to be of use to Rimuru. Hence, when Rimuru awaked he was grateful and allowed the Archdemon to become one of his comrades. When Rimuru found out he didn’t have a name he named him Diablo. And told him to become his guardian deity. A black cocoon enveloped his body creating a perfect evolution posture.

Saint Demon Confrontation Arc
It is later revealed that he is in fact not a Archdemon, but a Demon Duke. "I am not an Arch demon like you had previously said, but a Demon Duke." He reveals while battling a Templar Knight. A Demon Duke is a legendary existence. It is supposedly ranked higher than a self-proclaimed Demon Lord. There has been no other Demon Duke summonings previously documented.