The Grail – what is it?

Yes, I only wrote about the Grail skills, which are different in different castles, and about increasing the purchase of all creatures for a week and getting more gold per day; I hope this is clear to everyone.

Added after 23 hours and 19 minutes
I noticed another trick: the computer opponent starts "digging for the Grail" very early, and usually with a weak hero. If you notice where he is searching, just kill the opponent and dig in the same area. I usually don't waste time on obelisks...