Aggressive Horn is an Alliance War ability, found in the Assault Skill Line. A great Ultimate for Support roles.

Aggressive Horn
250 Ultimate Instant Cast Target: Self Base Skill: War Horn
Sound a war horn to rally your forces, increasing you and your group's Max Magicka and Max Stamina by 10% for 30 seconds. You and your allies gain Major Force, increasing your Critical Damage by 20% for 10 seconds.

Aggressive Horn is a morph of the War Horn base skill. The other morph is Sturdy Horn.