Played in a-quasi epic story with a BBEG named Zargon from Elder Evils. (Held back commenting too much at the table since I knew the backstory.)
Spoiler:
The DM was very clear, and repeated himself, about Zargon's regenerative powers. Basically he had to be beat then destroyed by traveling to a certain spot. Party was too used to being at full power all the time, and ignored the sense of urgency in favor of regaining spells. I tried to get them to hear the DM by reviewing what he said. After 3 times of being ignored I let it play out, and the rest period resulted in us failing to succeed.
Sometimes you can lead a horse to water...