If I do Mythic Nighthold, am I locked from doing it on Heroic for a different Transmog?
No. /10char
Awesome. Thank you!
No, for Siege of Orgrimmar onwards, normal, heroic and mythic all have separate lock outs.
Normal and heroic are loot based lockouts, so you can redo boss fights provided the group leader doesn’t done those bosses. Mythic however is purely ID based, so you can only do mythic once per week.
For older raids, you can do the raid once (naxx to throne of thunder) per week. You can LFR without locking out normal or heroic modes.