About the Skill:
The Focus skill could be defined as the "Warrior's Meditation Skill." It increases both a character's Mana Regeneration 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 statistics 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 hit points, stamina, and mana over time. These "10 second gains" 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 Focus skill.
For every 10 points of Focus a player has, their 10 second Stamina gain is increased by 1.
For every 20 points of Focus a player has, their 10 second Mana gain is increased by 1.
Focus is also a term used in the determination of the "chance to cast" spells and Bushido/Ninjitsu moves. This focus is quite different from the skill Focus.
How to train:
Focus is the easiest skill to train, even if you don't want to. Every time you regenerate 1 Mana or Stamina you have a chance to gain in Focus. Thus, simply do activities which cause you to lose mana and stamina. Using up mana can be accomplished by casting any type of spell. To lower stamina you can just run around a lot. Doing both at the same time will quicken gains. The Necromancy spell Pain Spike would be especially useful.
It is best to train Focus last because chances are it will be close to 100 by the time you focus on it.
It is also recommended that those training Focus should take the New Player Quest offered by Sarsmea Smythe The Focus Instructor at the New Haven Bank. A blessed reward, the Clasp of Concentration, for little extra effort is well worth seeking her out.