How does challenge the void work?

[John Jigglypuff]

It pops up sometimes but i can never get it constantly and its super confusing. can't find anything for it on the wiki either

[Yextiny]

It's a random chance that one will show up, and if you accept, you will get an extra boss to deal with and a reward to collect when you finish the level and go into the in-between stage

I believe the reward always includes a Max HP heart and gems, sometimes weapons and items too I think

[Dollop of Mayo]

Also, it doesn't stick around after you run past it, which I learned the hard way. When you hear that laugh, you gotta stop and decide if you want to do the challenge or not.

[John Jigglypuff]

thanks!