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Wondering what is known of the 3rd Moon (hidden moon?)
Otherwise, since the third moon seems to mostly feature in the lore regarding the Mane, it might be worth looking at the Reaper's March questline and Pridehome in Southern Elsweyr where the "forgotten Manes" lived.
there is this as well said about Azurah - The single moon Azurah carries is in her right hand, the stone of commitment mentions that souls were guided by the adepts of the hidden moon to the position of her left - which is her star (soul gem?)
I have to do the quest still in Southern Elsweyr - maybe there are more answers there..
edit:
and I can imagine this Moon being called the Necromancers Moon in the future (if it is the same third moon as was in the past era (hidden moon) and this era (dark moon)).