WorldEdit

What It Is

WorldEdit is a powerful Minecraft map editor that helps modify, create, and manage your worlds easily within the game. This Minecraft plugin will help you build large cities, custom designs, and make the process faster with tools like copy-paste, brushes, and schematics.

https://worldedit.org

Useful Commands

There's so much it can do - definitely read through their website before messing with it. It's extremely powerful, both for good and for evil.