dnd 5e – What is the spell save DC for the spells gains via the dark gifts in Curse of Strahd?

In Curse of Strahd, in Amber Temple,

the player characters can interact with amber sarcophagi and make deals with the vestiges trapped within them to gain dark gifts. Some of these dark gifts allow you to cast spells (only those in the X33 areas, found on pp. 191-193; the dark gifts gained from area X42 are different in nature and are not relevant to this question).

The descriptions of these dark gifts do not make any mention of a spell save DC, should the dark gift allow the casting of a spell. For a player who has a spellcasting class, I guess you can just use their class’s spell save DC (that’s what I ruled when this came up the other day, although if they had different spell save DCs if they were multiclassed, which one?), but for non-spellcasting classes, clearly they have no spell save DC from their class.

For some of these spells, clearly a spell save DC isn’t relevant, such as:

mind blank (X33b; North Sarcophagus) or raise dead (X33a; East Sarcophagus)

but for others, it is, such as:

lightning bolt (X33a; South Sarcophagus) or suggestion (X33b; South Sarcophagus)

but it doesn’t seem to mention anything about a spell save DC for these spells anywhere, either in each specific dark gift’s description nor in the general information about dark gifts in the sidebar on p. 191.

Am I missing something, or is there another way to determine what these spell save DCs should be?