How does "Book of Heroes" work?

A book of heroes is a magic item which is equal cost of 500 gems.

Claiming book of hero from season challenge

You can buy them from trader shop, Clan medal shop or can earn them for free after completing the season challenges.

Upgrading barbarian king and going to finish it instantly using book of heroes

You can also earn them by buying tje Goldpass. I have already claimed it by completing the season challenge.

Barbarian king will be upgraded instantly by book of horoes. No use of gems to reduce time

Book of heroes is one of thwtherefore e most efficient magic item ever. If You're reduces the time of any hero upgrade.

Barbarian king upgraded instantly by using book of heroes

If you have enough dark elixirs and upgrade a hero using a builder, it will take a couple of days or hours. But when you have a book of hero you can use to finish when you want without using gems.

If your heroes are max or you don't want to upgrade heroes then you can sell this magic item for 50 gems. This is a big loss for you though.