To complete this challenge, the player must activate the Obelisk in a hidden level only reachable via the Celestial Portal.
To open the Celestial Portal, a message has to appear in the chat box, saying "A celestial orb has appeared...". This only occurs when looping around (that is, after returning to the first stage) on Rallypoint Delta, the third zone. Once it's appeared, all the players need to do is complete the Teleporter Event to spawn the Portal.
Once the Celestial Portal is spawned, heading through it (not the Teleporter) will take the players to A Moment, Fractured. At the bottom of a chain of floating islands is the massive Obelisk. Having any items or abilities that improve mobility (e.g. Commando's Tactical Dive, Huntress's Blink, Milky Chrysalis, Hopoo Feathers) should help the players in reaching it.
At the Obelisk, two prompts are given (essentially, an "are you sure?" message), and the second activation immediately ends the run for all players. In exchange, everyone unlocks the Mercenary for all future playthroughs and gain 5 Lunar Coins.