Fire Mastery, one of the 8 available Masteries you can choose from.
Fire Elemental Mastery allows the ninja to learn Fire techniques that allow ninjas to dish out large amounts of damage to multiple ninjas.
Fire Mastery offers many advantages in terms of area control and damage. Most of the techniques available have very high base damage, range and AoE(Area of Effect). With this combination they are able to hit multiple targets at once for large amounts of damage. They are also able to cause a burn to deal damage over time. One small drawback to this is that some of their techniques have a slow travel time.
For PvE play, this mastery is top tier. With its damage output, aoe and range, it makes killing mobs easy.
For PvP it's not as great as the projectiles are a tad slow so it makes hitting a running target hard. In group fights that weakness is somewhat averted as you can just fire and will most likely hit someone.