Skarn, Herald of Terror is a Monster in Diablo Immortal. His exact role is currently unclear, but he appears to be in control of demonic armies and players may need to fight and defeat him as a boss.
"The demon known as Skarn, Herald of Terror—once Diablo’s most powerful lieutenant—is rallying a new demonic army. Skarn intends to gather the fragments of the corrupted Worldstone and use them to resurrect Diablo, reigniting the Eternal Conflict between angels and demons.
Skarn is not the only one hoping to seize the window that’s been opened by the Worldstone’s destruction; a fledgling Coven is at large, and dark cultists roam the land, spreading corruption wherever they go."