The Aberrant Mage Sub Mission in Dawn requires special objectives before it’s unlocked. This guide on Where To Find 10 Onmyo Mages In Nioh 2 will tell you where you can find and quickly farm the Onmyo Mages you require to unlock the Aberrant Mage Sub Mission.
This Sub Mission is part of the Dawn chapter of Nioh 2. It’s locked behind two primary objectives that you must first complete. The Sun Sets on Mount Tenno is a story-based mission that must be completed, along with the following story missions, before any Sub Missions become unlocked in the Dawn chapter. Once unlocked, the Aberrant Mage appears with one further objective. Defeat 10 Onmyo Mages in Nioh 2.
Where To Find 10 Onmyo Mages In Nioh 2
For this mission you need to return to The Sun Sets on Mount Tenno. Progress through the mission until you reach the second Kodama Shrine, it’s at the rice paddies with a lot of human enemies in the area. Continue past that point until you reach a small bridge, as shown in the image above. Instead of crossing the bridge, you need to drop down and enter a nearby cave. Inside the cave are two enemies that use magical attacks, like slowing you down and weakening your armor or attack.
Kill both of these mages and then return to the Kodama Shrine – this is much faster if you open the appropriate shortcut so you can return to the Kodama Shrine in the fastest possible way. Using the shrine respawns enemies, including the Onmyo Mages. Repeat these steps until you have killed 10 Mages and then you can leave the mission and The Aberrant Mage Sub Mission will now be available for you to complete.
If you are playing with a friend, using the shrine does not reset the enemies. They will need to quit the mission each time you clear the mages, and then return after the host has used the shrine again to respawn the enemies.
from GamersHeroes http://www.gamersheroes.com/game-guides/where-to-find-10-onmyo-mages-in-nioh-2/