Now and again, I sometimes hear about players doing this endgame stage, is it the absolute ending of the game or something?

Anyway, I was wondering about the coil or turns of Bahamut, which is first and when does it start to happen (despite Lv50 being the primary requirement)? With ACN at 34, Im miles away from doing this sort of thing.

Can someone explain this part of the game, if thats ok?