When I cast a spell from a magic item, how do I set the spell save DC?
Set the DC the same way you would when casting a spell without using a magic item (DC equals 8 plus your spellcasting ability modifier plus your Proficiency Bonus). If you don’t have a spellcasting ability, your spellcasting ability modifier is +0 for the item, and your Proficiency Bonus still applies.
Should this have "Unless the magic item otherwise specifies it"?
For example Necklace of Fireballs has a specified spell save DC of 15. Or is this because you're not 'casting a spell', but rather 'throwing a pearl' which then casts the spell?
There's a general principle in D&D rules that specific always beats general. Given that, it's not strictly necessary to ever say "unless otherwise specified", though I agree it would make things a little clearer.
If it doesn't specify a DC, then assume you're using your own Spell Save DC.
Most magic items that require the use of your spell save DC will tell you what to use, even if they just say "use your own".
There are a few that dont.
Hat of Disguise, for example, does not. But you also dont use your Spell Save DC when casting the spell initially. But still requires it for someone to see through the disguise.
From the new Sage Advice Compendium:
Should this have "Unless the magic item otherwise specifies it"?
For example Necklace of Fireballs has a specified spell save DC of 15. Or is this because you're not 'casting a spell', but rather 'throwing a pearl' which then casts the spell?
Another more "cast a spell" example is Wand of Polymorph.
There's a general principle in D&D rules that specific always beats general. Given that, it's not strictly necessary to ever say "unless otherwise specified", though I agree it would make things a little clearer.
pronouns: he/she/they
If it doesn't specify a DC, then assume you're using your own Spell Save DC.
Most magic items that require the use of your spell save DC will tell you what to use, even if they just say "use your own".
There are a few that dont.
Hat of Disguise, for example, does not. But you also dont use your Spell Save DC when casting the spell initially. But still requires it for someone to see through the disguise.
Since it doesn't specify, you use your own.