Ferocity is one of the most important unit attributes. 
It determines how likely your unit's extra damage can be dealt with during battles.

Important Information:
The higher the Ferocity attribute of your units, the higher extra damage the units can deal.
Ferocity applies to all unit types: Infantry, Cavalry, and Archers.
Ways to increase Ferocity include:
Completing research in the Academy.
Unlocking talent related to Ferocity from the War talent tree.
And other methods."