You have to complete all 4 rituals, and the Pack-a-Punch ritual in the Rift.
You need a party of four people who know exactly what to do, so communication is key. Finding a group of four can be difficult on it's own, as last-gen Black Ops 3 doens't have split-screen.
Doing the four 'normal' rituals also grants you the 'The Beginning of the End' trophy.
Here's a video showing how to do this, and who needs to do what and when:
*EDIT 5-1-'16*
So more than one and a half month after the release, there has finally been a way found to do this on solo. There already was another way for next/current gen, but not for us last-gen. Until suddenly BUBBLEGUM commented on this guide that there is a way to do it on solo. He created this strategy and also created the video, in which he showed how to do it. It's this one:
The quality isn't great, but it's still clear what to do. The trick is doing both the Junction and the Waterfront districts in one Beast Mode. There is barely enough time to do it, so if you screw up for one second, you gotta try over. This Beast Mode took me about 4 hours until I managed to do it.
Shooting the electric box to open up Nero's Landing seems to be either glitched, or just very, very hard to do. That's what you got to practise the most.
Good luck!
Special thanks to BUBBLEGUM for providing us with the solo strategy!