In the light novel, it's instead implied (and pretty much made official in volume 7) that Diablo didn't evolve from being named by Rimuru. He, like the all the other Primordial/Progenitor Daemons, was already a Daemon King class daemon (which is essentially the Demon Noble rank above Demon Duke). Heck, Moss, one of Testarossa's lieutenant, was already a Daemon Duke prior to being named in the light novel.
They willingly sacrifice themselves to give Rimuru enough magicules to revive Shion and co.
@LoLzM Finally someone who asks this question correctly (for the light novel rather than the web novel, since the translation of the light novel is already beyond the manga). To answer your question, I just checked in my physical copy of LN volume 5, and this manga chapter ended on page 233 of chapter 5. You don't need to buy the physical copy or the ebook though, since it's easy to find pirated PDFs of the YenPress ebooks online.
As for volume 6, I also recommend the YenPress translation for it (since the fan translation for it isn't that good and has many mistakes in it that aren't being fixed due to the translators prioritizing the later volumes).
Then, for volumes 7 and 8, I recommend the fan translation, which can be found here: https://tensurafan.home.blog/light-novel/