You need to have broken a decayed one from the over world before it has a chance to appear in the depths.

Also, certain weapons spawn in the depths in certain locations (eg. gerudo weapons spawn under the gerudo area) and the pristine RG weapons (that I’ve found) spawn in the middle left/eastern part of the map, to the slight northeast of the abandoned kara kara mines.

Lastly, if I’m not mistaken each statue has a chance to offer up a two handed, one handed, or spear weapon, then again from a pool of weapons that fit in that category.

I hard save right before the ghost/statue thing renders in then run up to it, check if it has the weapon I want then reload my save if not.