Diablo/Chronology

Demon Lord Awakening Arc
By offering the soul-less corpses of the remaining 10,000 Falmuth soldiers, Rimuru successfully summoned Noir and his two Greater Demon subordinates. Using the corpses, they were able to successfully materialize and Rimuru's first order for them was to capture the hiding Razen and bring him to Ranga. Mesmerized by Rimuru's Soul the three Demons did their work promptly, successfully capturing Razen alive.

When the secret revival art was lacking the necessary energy, Noir offered to sacrifice his subordinates, stating that they were delighted to be of use to Rimuru. Hence, when Rimuru awakened and found out that the Arch Demon he summoned did his job properly, he was grateful for his efforts and allowed Noir to become one of his vassals. When Rimuru found out that the Arch Demon didn't have a name, he named him "Diablo." After Diablo was named, a black cocoon enveloped his body, and the now named Diablo evolved into a Demon Peer.

Afterwards, Diablo was allowed to attend the meeting to discuss their future plans.

The Octagram Soars Brightly Arc
Diablo was then given the task of investigating Falmuth to cripple its economy and overthrow its current king.