The most accuracy (for BLU) on a back piece in the game. Even if you prefer Amemet/Forager's/Cuchulainn's for your main TP cape, the magic accuracy is not to be dismissed.
These can also drop off Nightmare Dhalmels in Dynamis - Buburimu.
It is believed that magic accuracy plays a role for what concernes additional effects on physical spells, therefore making this the best Head Butt mantle there is.
Very acceptable for TPing and 5-hit spells if you don't want to spend 5m+ on C.cape as well.
Finally, this is possibly the best mantle for landing any debuff such as Soporific and Frightful Roar; perhaps INT+MND on Prism Cape boosts your accuracy on those spells more than this piece does, but I don't think there's any confirmation on that for now.
As well as Nightmare Sheep and Nightmare Dhalmel, this mantle may be dropped by Nightmare Manticore in Dynamis - Valkurm and Nightmare Bunnies in Dynamis - Buburimu.