Potion Protection by aciid.
Potion Protection gives players an easy way to create worldguard regions
simply by throwing a potion.

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PotionProtection allows you to assign cuboid shaped WorldGuard regions (configurable height) to any potion so that when that potion is thrown by a player, a WorldGuard region is automatically created at the impact location. You can define the size of the region, the potion to use, and with permissions you can limit the maximum amount of potion regions players can create in each world, and with what potions, you can add automatic worldguard flags, spawn a fence or border around the region, and more!
When configured properly this plugin will not interfere with gameplay because there are a handful of unused potions which PotionProtection can be set to use (these potions can not be crafted by players and have no effect ingame).
To stop players from building in a world unless they are inside a region you must
use a __global__ region. This is a region that spans accross an entire world.
To set the global build flag, login to the world and type the following in-game:
/region flag __global__ build deny
Note the double under_scores.
This will add the __global__ region to your WorldGuard/worlds/world/regions.yml
Please report any bugs/issues, explaining what you were trying to do, what happened, and include all error messages!