It follows the concept of the beginning of the Crota's End raid. It is a round-based survival game mode where you start at point A and have to get to point B via the "lamps". You obviously get chased by the thralls and some other enemies like in the raid. Only one person in the fireteam has to get to point B. Once the point is reached all dead teammates respawn at this checkpoint and the next round begins. As the rounds progress the game gets harder - more enemies, longer runs, greater distance between lamps, more holes in the ground etc. After every 10 rounds a "bonus boss" can come in which the fireteam then have to beat to move onto the next set of rounds. After the boss is beaten the fireteam can get loot and also a modifier can be added, so for example you do 9 rounds of running until round 10 when you face a boss, beat it, get loot then move onto the next 10 rounds but this time the Heroic modifier can be on. After round 20 both Herioc and Angry can be on etc etc.