Who is Akarat in Diablo 4?

Who is Akarat in Diablo 4? If you've played Diablo like, at all, you'll hear his name a lot. So if, like me, 'Akarat chirs cha cham' is playing over and over again in your head, here's a full rundown of who your godly companion is, and where he fits in the Diablo story. Note that this guide contains spoilers for the Vessel of Hatred campaign.
We've all seen the trailer for Diablo 4: Vessel of Hatred. Neyrelle, guided by a native of Nahantu, crosses the river to Kurast in a rowing boat, dutifully chanting to Akarat. As Mephisto takes over and her arms are mercilessly split apart, we realize that Akarat is going to play a huge part in the RPG's campaign - which we have a Diablo 4 DLC review for, if you're interested - especially given the breadth of Mephisto's inky corruption. If you're new to Diablo, however, you may not have encountered Akarat, so here's a rundown of exactly who he is, and what part he plays in Sanctuary's story.
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