Mystical Spirit Powder usage is to Recover durability of alchemy stones 

How to get the Mystical Spirit Powder :

First You need a spirit stone like Destruction or Guardian or Life Spirit Stone

You can get them from grind spots in kamasylvia or you can buy them from market place :

Also you can Exchange 1 Laila's Petal to get Mystical Spirit Powder x 5 ''Even though it isn't worth it''

After you get the spirit stone you need to use Grinding to process it :

Like that you will get the Mystical Spirit Powder 

I'll give an example of how to repair your alchemy stone with it

Open your inventory then left click on the Alchemy stone icon

Now we are going to recover our alchemy stone durability by Recharge it with Mystical Spirit Powder

Any craftable Alchemy Stones can be recharged

If  you have any question related to  BDO   send me a message in game my family name is :  Svax  

or in discord :  Svax1

Have a good day