> ###Luck Blade
> [...] While holding it, you can use an action to expend 1 charge and cast the wish spell from it. (DMG 179)

Based on this it is you who is casting the spell. Since the item does not force you to use a certain DC, you should be using your own spell DC, or rather 8 + spellcasting ability modifier + proficiency bonus

Per the DMG, on page 141:

> If you don't have a spellcasting ability [...] your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.

So it is either **your spell DC OR 8+proficiency bonus**.