Out of curiosity, I’ve been calculating what is the highest spell save DC that a character can have, assuming perfect conditions.

My calculations have so far yielded the following result:

• Base of 8
• Proficiency bonus of +7 (level 17+ character attuned to an Ioun Stone of Mastery)
• A charisma of 24 (to my knowledge, there is no way pf having a higher charisma than that, and the only way I know to get a 24 of charisma is thanks to a specific event in a specific hardcover adventure), giving a +7 bonus
• Attunement to a Rod of the pact keeper +3 (assuming that the character is a warlock)
• Attunement to some Robes of the Archmagi, giving another +2 bonus

Total: 27

Am I missing something? Perhaps there is yet another magical item that can increase a character’s spell save DC (assuming that the item in question requires no attunement, or that the character somehow found a way to gain a fourth attunement slot)? Or perhaps a spell, feature, feat, ability, etc.? Or perhaps there is indeed a way to get a 30 of charisma?

The calculation assumes that the character is to be played in Adventurers League v.7 organized play (so no homebrew).