Arcana Excelsis Initiate
This quest in Arcana Excelsis leads to the insignia Arcana Excelsis Initiate, which modifies mana regeneration by + 3% continuous until hero level 250 (the mana regen bonus goes away at 250, but the insignia itself is still there - see note below).
You need to be at least level 35 and able to see invisible objects to find the entrance to Arcana Excelsis. To see an essential mob you need to be at least level 39. This quest is completable before hero level 200 (possibly higher, though 447 is too high).
Start by bowing to Itania.
Note that you will not be able to start the quest if you have the Arcana Excelsis Initiate insignia from a previous incarnation (it does not disappear on remort).