Book: Crucible Playlists.
6v6. Cyclical objective-based team combat. Battle to capture the Spark and run it to your enemies' Rift to score. Hunt down the Runner and shut them down to reset the Spark and attempt a new run.
In trying times, the Vanguard must look inward for new ways to prepare their Guardians for the threats that face the City. The newest introduction to the Crucible came about when a little-known experimental Warlock focus exercise was discovered by Ikora Rey and modified as a sanctioned training drill for all varieties of the Light.
The highest members of the Warlock orders are too proud to acknowledge it, but it's clear a few resented giving up their private game of light-sport. Anything in the name of the Traveler, of course, but If there's anything a Warlock loves, it's a good secret.