It is the personal tome of Olwen (Righteous Knight).
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. In recent updates, it obtained a special refine, giving Olwen a supportive role as a debuffer, inflicting -5 of speed and resistance to foes within 2 spaces of Olwen and her nearby ally, when her or said ally starts combat.
Adds total bonuses on unit to damage dealt.