Boosts damage dealt by 50% of unit's Res.
There are several specials with damage based off defensive stats in Fire Emblem Heroes. Bonfire and Ignis activate off Defense, whereas Iceberg and Glacies activate off Resistance. Chilling Wind and Glowing Ember are the non-upgraded versions of these skills. Ignis and Glacies are the stronger of the two, trading slower charge times for higher damage. Bonfire and Iceberg are more often intended for use in a duel, thanks to their reasonable charge time. Provided the unit’s Defense or Resistance is high enough, they are a great general-use special. Ignis and Glacies are powerful nukes, designed to tear through enemies in a single hit with their huge 80% stat modifier. For example, a unit with 35 Defense will deal an additional 28 damage with Ignis.
The first and only real requirement to run these specials is a high Defense or Resistance. The higher the stat, the higher the damage. Preferably, the unit should also have a high Attack stat in order to deal more damage. These specials are often run as part of a “bomb” build run on tanks, intended to charge up a powerful special and use it to OHKO a fresh opponent. These builds are very potent in the right hands, and allow them to threaten nearly anyone on the battlefield. Slaying or Killer weapons are popular choices thanks to their special charge decrease. Common skills seen on these builds are Fury, Quick Riposte, Renewal, Vantage and Distant Counter. Less-desirable skills such as Atk/Def+2 and Fortress Defense can be used effectively in tandem with these specials. Skills such as Close Defense, Armored Blow and Distant Defense all work with these specials, giving them an even larger boost.
Users of these specials must be KO’d before they have a chance to unleash them. This is possible with fast-charging specials such as Moonbow, which conveniently ignores a percentage of a foe’s defense when attacking. Having a Luna user holding the Quickened Pulse seal is an effective method of countering these specials. Once an enemy has it charged, if they have Vantage and/or Distant Counter, it can be nearly impossible to avoid losing a unit. The Hardy Bearing seal can bypass Vantage, as can a Vantage user of your own (by luring them to attack you).