So what happens is, when you USE one, it converts an existing boost in your inventory (such as a Strength Booster) into a “Enchanted Combat Boost”.
The “Enchanted Combat Boost” can be one of 4 types:
Strength Booster
Speed Booster
Armor Booster
Rejuvenation Booster
Another type is a Enchanted Reward Booster; it takes the following boosts and combines them into a single booster type:
Experience Booster
Magic Find Booster
Crafting Booster
Karma Booster
Gathering Booster
This basically allows you to stack different booster types in one inventory slot. When you use the “Enhanced Combat Booster”, or “Enchanted Reward Booster” you pick which boost you want and it imbues you with it directly.
So the Powder doesn’t really do anything by itself; you have to use it to turn your old/existing boosters into the new “combo” type boosters, which saves on inventory slots.
