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Aurum Vale is not part of the main story and is a side quest given by the NPC Nedrick Ironheart in Vesper Bay (Western Thanalan). The quest is given at level 47 and the max level for this dungeon is 49. Any player above 49 will be level synced to 49 upon entry to the dungeon. The dungeon is located in Coerthas Central Highlands.
I still haven't bothered running it yet. Didn't stop me from finishing the story, doing endgame content, maxxing my GC rank etc.
This dungeon is pretty good to gain some good experience when you reach the part with the things that keep on spawning mini marbolos until destroyed.
With guild exp buff, food and chain on killing them I was reaching on 2k exp per mini marbolo and they die fast.
AV is one of the two ways to unlock AK; you can also do it through the GC quest line if I recall it correctly.
So no, you don't have to do it, and you can skip it, but for a L47-49 character it's a pretty nice xp run dungeon, and it has some new mechanics which make it fairly interesting to run.