One of the options for Nystul's magic aura is:
Mask. You change the way the target appears to spells and magical effects that detect creature types, such as a paladin's Divine Sense or the trigger of a symbol spell. You choose a creature type and other spells and magical effects treat the target as if it were a creature of that type or of that alignment.
This all seems to make sense except the part at the very end about alignment. You only choose a creature type, not an alignment, when you use this option of the spell. In fact, this is the only mention of alignment anywhere in the spell's description, so the fact that it says "that alignment" as if referring to a previously mentioned alignment makes it seem like this could be a part of the text from an earlier revision that was accidentally left in after all other mentions of alignment were expunged.
So, is nystul's magic aura actually intended to allow masking of alignments as well as creature types? If so, how does that work? Do you choose an alignment as well as a creature type, or do certain creature types automatically imply certain alignments (e.g. devils and angels)? If not, is this an error in the spell's text?
Best Answer
In 5e, alignment has been greatly downplayed and marginalized. Spells that used to detect alignment now detect creature type. However, some creature types still have an alignment associated with them. Celestials are good, usually lawful. Fey are chaotic, usually good. Fiends and undead are evil. Beasts and other creatures with low intelligence are unaligned.
Thus, the last line you quoted appears to be a carry over for DMs that want to continue to use alignment.
An example usage:
If you wanted to hide a Celestial from a Paladin, you could cast magic aura, using its Mask effect, and choose Beast as the type. Your Celestial (Good) would now appear to be a Beast (Unaligned) to the Paladin's Divine Sense and be undetected.