So I got a luck blade from my DM and I am trying to cast dominate person on someone at 8th level. What would the saving throw be for the person it is being cast on. Wish says that any spell at 8th level or lower can be immediately cast and doesn't state it has to be at its lowest casting form. Would you do the dc 18 of a scroll or is it 10+spell level+ min ability score. This is all in 5th edition and I can't seem to find anything that deals with this sort of scenario. The dm has allowed it be at 8th level since it a wish spell.
It's unclear, but effects produced by Wish can be treated as if cast at 9th level, and looking to the rules for scrolls for a guideline on what saving throw DC/attack bonus to use for such spell effects is reasonable - DC19 in this case.
When a magic item in 5e has the ability to cast a spell, it is always done using the lowest possible level. From the DMG:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise.
However, Wish is a 9th level spell and can only be cast as such. In this question it is accepted that the spell effect duplicated by Wish is done as if a 9th level slot is used. Further, this tweet by Jeremy Crawford suggests that it can produce effects at 9th level and the caster could choose what level to produce the spell effect at.
Brail: Use "Wish" to duplicate lower level spell.Is it considered to have been cast as a 9th level slot?
Jeremy Crawford: I'd let the caster decide.
So your spell can be cast as if using a 9th level slot if you like.
Working out the DC for a spell cast by a magical item is trickier. There is no hard rule for this. Some items, intended for use by casters, use your own spellcasting abilities to determine their DCs:
A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability - perhaps you're a rogue with the Use Magic Device feature - your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.
However this is always called out in the item's description, such as the various staves which all explicitly say the spells are cast using your save DC. For other items, the DC at which they produce their effects is stated in the description. The Luck Blade is a bit of an anomaly because it has such a variable effect and the need to know what DC it might end up using was not considered.
Your best bet is probably to treat it as a scroll producing a 9th level spell, so having a save DC of 19 (or an attack bonus of +11, if you duplicated a spell that makes an attack roll). The alternative (using your own caster abilities) would give most characters who might wield it a really pathetic result, which almost certainly is not intended for a legendary item; and given it is a legendary item with extremely limited charges, it's totally reasonable for it to produce a powerful effect.
[...] 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.