Return of Reckoning

Could you tell me if the reward for defeating each boss in Hard Mode is only available once, or does it reset after a certain amount of time? Thanks.

Currently awarded for the first clear only.

Starting with Bastion Stair, we've begun adding achievements for clearing a Hard Mode boss five and ten times as well. Once we finish Bastion Stair soon, we'll likely go back and add this feature into Gunbad too.

thanks for the answer

what is the reward? how do you trigger the hard mode?

IIRC champion coins. You trigger them by interacting with an item in the respective boss rooms / areas.

Champions marks reward. We completed the hard mode several times, but only received the coin once, so the question is, is there a rollback like with a speedrun (maybe a week).

No, they are bound to achievements. For now, you only get them once.