There is an item in the Waterdeep: Dungeon of the Mad Mage adventure, the
described on p. 284. Its description states:
As an action, a creature attuned to the black crystal tablet can use it to cast eyebite...
However the eyebite spell states:
One creature of your choice within 60 feet of you that you can see must succeed on a Wisdom saving throw or be affected by one of the following effects of your choice for the duration...
What is the spell save DC used when casting this spell?
Note that the entire description of the item (provided below) does not list a save DC to use when casting spells from it:
The black crystal tablet is a legendary magic item that requires attunement by a creature that has proficiency in the Arcana skill. Any creature that attunes to the tablet must make a DC 20 Wisdom saving throw at the end of its next long rest. On a failed save, the creature becomes afflicted with a random form of long-term madness (see "Madness" in chapter 8 of the Dungeon Master's Guide).
As an action, a creature attuned to the black crystal tablet can use it to cast eyebite or gate (the portal created by this spell links to the Far Realm only). After the tablet is used to cast a spell, it cannot be used again until the next dawn.
It is notable that this item does not require you to be a spellcaster to attune to it, so even creatures without spell save DCs could attune to it.