Buser was a demi-human tribal leader who ruled over the Bafolk Tribes in the Abelion Hills. He's best known for his involvement in Jaldabaoth's invasion of the Roble Holy Kingdom, where he fought alongside other tribal chiefs under the demon's command.
He didn't survive the campaign, though. Ainz killed him, reportedly because Ainz believed that Pestonya and Nigredo would find his habit of wearing child skulls as trophies deeply disturbing. It's a pretty dark detail that illustrates just how brutal and extreme some of the invasion's participants were.
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