How to get the true ending stone?

[guille]

He always says I left one boss alive but I swear I kill one in each level (even the Slime King).

[Gonna_Fly]

the slime king doesn't count, when starting a run press the Pause button and look on the badges of boss the ones that aren't crossed out.

[Xeapnea]

Yea if youre looking to finish the game you need to go down the other exit in caverns 4, you have to enter the golden core which will take you to the seer fight, that is where you get the last fragment.