Speed Thunder (ブーストサンダー Būsuto sandā, lit. Booster Thunder in the Japanese version) is a Tome that is introduced in Fire Emblem Fates. A rare tome, Speed Thunder is comparable to Fimbulvetr, but lacks an Attack Speed penalty. Engulfing a selected enemy target in a cage of blue lightning, Speed Thunder boosts the Speed of nearby enemy units, and is thus only really viable as a last-ditch attempt to kill stray enemy units.
Enemies within two spaces receive +4 Spd after use in combat.