What is this?
Mystery Amulet Circle is an Easter Egg in Onmyoji. It only triggers when you do a rare summon (either by Amulet or by Jade) by drawing a special pattern. The pattern resets every month and each player can only claim the prize once.
The reward is a Rare Summoning Amulet so this means at the end of the day, you get one rare summon for free.
March Mystery Amulet Circle is a straight line across the screen just below the two banners.
The Mystery Amulet Circle for this month is a straight line from top to bottom in between Kagura and the Amulet.