Dead Cells

The Challenger Rune can be unlocked be defeating the first boss named "The Concierge" located at The Black Bridge.

To get to The Black Bridge, you must take 1 of 2 routes:
The Prisoner's Quarters - Promenade of the Condemned - Ramparts - The Black Bridge
The Prisoner's Quarters - The Toxic Sewers - The Ossuary - The Black Bridge
This boss isn't too bad. You'll want to use things like traps, turrets, and effects to take care of him. Keep an eye on your health, and dodge when necessary. Defeating this boss will cause him to drop the Challenger Rune, automatically equipped as a permanent unlock, as well as unlocking an achievement.

With the Challenger Rune equipped you can now access Daily Challenges from the starting area or the main menu. The daily challenge is a competitive leaderboard mode finishing in a boss battle. Completing a run gives you progress towards unique blueprints and unlocks an achievement.

You have to reach the black bridge and defeat the Forst Boss "The Concierge". He drops the challenger rune. It looks like a clock.

Have Fun!