Mythic Nyalotha Shadhar the Insatiable – 24th Feb 2020.
Other bosses in this tier:
Mythic Wrathion – downed 30th January 2020, also Server 1st! (we forgot to screenshot but enjoy the video)
Mythic Maut (no screenshot /cry)
Mythic Dark Inquisitor Xanesh
Mythic Shadhar the Insatiable
Mythic Ra-den the Despoiled
Mythic Carapace of N’Zoth