This depicts Orcus raising Cyrog from Out of the Abyss
Warning: this answer is full of spoilers
During the Out of the Abyss adventure, the party travels to a location called Gravenhollow, a library that, among other things, allows the party to experience visions of events of the past, present and future.
The relevant section from Out of the Abyss is "Cyrog Lives! Hail Orcus!" (pg. 158), which details one such vision of Orcus turning up next to the recently deceased Elder Brain "Cyrog" and bringing it back from death, then Cyrog tells the colony that "Orcus has saved Cyrog" and commands them to "follow it [Cyrog] into undeath".
This doesn't actually play much of a part in the adventure, save for providing a plot seed for the DM to run with, but that's what that image at the back of Volo's Guide to Monsters is showing.