What is the spell attack bonus and spell save DC of a Thief using the Use Magic Device feature?

Question

A 13th level Rogue with the Thief archetype has the following feature:

Use Magic Device

By 13th level, you have learned enough about the workings of magic that you can improvise the use of items even when they are not intended for you. You ignore all class, race, and level requirements on the use of magic items.

And the DMG has this item, on page 211:

Wand of Paralysis (requires attunement by a spell caster)

Make a ranged attack roll against the creature using your spell attack bonus. On a hit, the target is paralyzed for 1 minute. At the end of each of the target's turns, it can repeat the saving throw.

So what is the spell save DC of a Rogue using their Use Magic Device feature, and what is their spell attack bonus?

A simple answer might be that Use Magic Device simply does not allow one to attune to this item.

I am personally not sure what to rule. My gut tells me it should be the player's choice of Intelligence or Wisdom, while still being on the fence as to whether they should be proficient or not. An alternative ruling would be just setting it to a DC of 15 like just about all the other wands and giving it a normal ranged attack roll via Dexterity.

Answer 1

you ignore all class, race and level requirements on the use of magic items

and

using your spell attack bonus

From p.207

Some spells require the caster to make an attack roll to determine whether the spell effect hits the intended target. Your attack bonus with a spell attack equals your spellcasting ability modifier + your proficiency bonus.

A spellcasting ability modifier is only given as a specific class ability. Barring multi-classing, a 13th level thief does not have a spellcasting ability modifier so this is 0. They have a proficiency bonus of +5. So +5. (DMG p.141)

Similarly for the saving throw.

Give it to the wizard!

Answer 2

In general, the spell attack bonus and save DCs of a Thief with Use Magic Device is given by the following rule (DMG, page 141):

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, in the specific case of the wand of paralysis, the entry in the DMG has been errata-ed.

“The target must succeed on a DC 15 Constitution saving throw or be paralyzed for 1 minute.”

So instead of a spell attack roll, it should be a DC 15 Con save, repeated at the end of each of the target's turns.