Outast’s Secret drop chance is 5.25% according to GD database.
I got it after 25 attempts, each attempt takes ~3 minutes. It definitely isn’t the most painful thing to farm in the game ( and its good roll isn’t so critically important as Decree etc. ). Just a bit boring, need to get her as Nemesis first…
Sounds like you got lucky, someone some time ago said they had done 300 runs after reaching Nemesis without seeing an Outcast’s Secret (but saw multiple Outcast’s Hoods), or perhaps they were just really unlucky? It’s all RNG 
No, it’s not mandatory
The mandatory gear for Full Aether Conversion is The Magelord Rings and Decree of Aldritch
For Helm, you can use Outcast’s Hood, non mythical Outcast’s Secret, Iskandra’s Hood, Clairvoyant’s Hat, or Maw of Despair
Using this One also gives you Nice Crit Damage, Armor, and OA (Complete set bonus)
Just use the Outcast Hood you got
It still works 
very nice build~thanks~^^
Thank you very much for this guide! I’m new to the game and have a question that I hope you could answer: is there a way to convert Storm Totem into Aether, if I don’t have neither The Magelord Rings nor Decree of Aldritch?
There are no items currently that do Lightning to Aether conversion so you’re stuck with Elemental to Aether conversion items. Besides the ones you mentioned, there’s only a few other options (which cut into the M. Iskandra set).