Artoomis said:"Unnamed" spell?? The spell is indeed named, and it's Nondetection. Unless, of course, the DMG or errata lists another spell, which I did not check.
It does not really matter, anyway; the description of the ring makes it clear that alignments cannot be detected. There is no reason to go back to the underlying spell descriptions as that is NOT what the item does anyway.
I do not see any analysis of the Undetectable Alignment spell as being value-added here. Am I missing something?
I think you're not missing anything. However, since the spell "detect thoughts", "discern lies" exists, I'll believe this is errata stuff they missed. The spells to create the item should be detect thoughts", "discern lies" and "undetectable alignment". This is no RAW, it's my opinion. (I thought it was RAW but I was shown to be wrong...).