True Respite

Obliterate yourself at the Obelisk.

Guide
To complete this challenge, you must activate the Obelisk in a hidden level only reachable via the Celestial Portal.

To open the Celestial Portal, you'll first need to wait for a message to appear in the chat box that says "A celestial orb has appeared...". This seems to only occur when Looping around (that is, after returning to the first stage) and most often on Rally Point Delta, the third zone. The exact spawn criteria for the orb are unknown as there seem to be several (such as time on Rally Point Delta, or killing a boss there), but either way once it's spawned all you need to do is complete the Teleporter Event to spawn the Portal.

Once you've spawned the Celestial Portal, head through it (not the Teleporter) and you'll be taken to a secret area. At the bottom of a chain of floating islands is a massive Obelisk; jump down carefully until you reach the Obelisk. If you have or can get any items or abilities that improve mobility (e.g. Commando roll, Huntress blink, Milky Chrysalis, Hopoo Feathers), they should help.

At the Obelisk you'll be given a prompt that you need to activate twice (essentially, an "are you sure?" message), and ''' the second activation immediately ends your run. ''' In exchange, you unlock The Mercenary for all future playthroughs.