Reduce experience from quests

I propose to create something (buff, scroll, skill) to reduce the experience gained for quests. Not everyone wants to get to the top level and finish the game faster, I want to play the game as long as possible.

Also do something to reduce the character's experience.

"You can achieve this via experience modifier in general tab of skill book."

Thanks, but I can't find where it is. Where is it located and how does it work? Help me please.