If you want spoilers on how he died then I can tell you.
I'm not sure though if you'll be satisfied but I am super satisfied
Spill it sister, just put an spoiler tag
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If I remember it correctly, his immortality was taken away by the MC, (the reason for immortality was due to the stone they are trying to restore in the early chap) he was like melting and transforming to a very old man when his daughter? Or grand daughter stabbed him with the sword.
It was revenge actually since this king is so rotten that he order all male descendants to be killed and have the females be his slave.
I don't want the king to die yet, pls I need the total number of people he killed and his subordinates to bash his head until it became a lump of pudding