Thunderhead

Thunderhead is a Legendary Tome and a weapon skill that appears exclusively in Fire Emblem Heroes.

It is the personal tome of Olwen (Righteous Knight).

Overview
Similar in function to a Gronnblade; Thunderhead adds all stat buffs to Olwen's Atk stat before calculating damage. In comparison to the inheritable variation, Thunderhead has increased power and lacks the special trigger delay.