Focus skill has two main functions; it could be defined as the “Warrior’s Meditation Skill”, it is also the support skill for Mysticism, enhancing the mystic’s spells in the same way as Evaluate Intelligence skill supports Magery skill.

It increases both a player’s Mana and Stamina regeneration.
Since a warrior’s swing rate is determined by the amount of stamina he has, the ability to increase the regeneration of that attribute would be highly beneficial on the battlefield.
Mana, too, is an important stat to any warrior that uses special combat moves as part of his tactics.

Hit Points, Stamina, and Mana all have stats associated with them called “10 second gains,” which are set on all creatures and tell the game system how much to increase each current value by every 10 seconds. This can be seen in the way monsters regenerate their stats points over time. These stats are usually set once and never change.
However, in Age of Shadows, these 10-second gains can now be increased by various skill and magic effects, like the Battle Focus skill.

For every 10 points of Battle Focus a player has, their 10 second Stamina gain is increased by 1.

For every 20 points of Battle Focus a player has, their 10 second Mana gain is increased by 1.

How to Use:

Similar to the mage’s Meditation skill, Battle Focus will raise passively as you regain Stamina and Mana. You simply need to do anything that lowers your Stamina and Mana in order to increase the level of your skill.