I am the wellspring of darkness, fed by Zemus's unbridled hate... I am He who is called Zeromus... I am he who knows naught but hate!
Zeromus is the final boss of Final Fantasy IV and the spirit and hatred of Zemus incarnate. Zeromus appears with three battle scripts, with only the third being possible to defeat. The party faces him in the Moon.
After Fusoya and Golbez defeat Zemus, Zeromus emerges from his corpse, proclaiming itself as Zemus's hatred. Fusoya and Golbez's attacks have no effect on Zeromus, and when Golbez tries to use the Crystal, the creature proclaims evil hearts cannot wield it, and attacks both with Meteor.
Everyone left at Mysidia prays for Cecil. Their prayers revive him and his party, and Golbez hands over the Crystal and beckons Cecil to use it. He does so, and Zeromus's invincibility disappears, revealing its true form.
Once Cecil uses the Crystal, given by Golbez, Zeromus becomes corporeal and the true battle begins. In the SNES, PS, GBA and PSP versions, Zeromus has considerable prowess in magic; his signature moves include Big Bang, a powerful magic attack that deals around 1,900-2,600 damage to each party member and inflicts Sap. His Black Hole attack nullifies the party's magical benefits (it does not affect Zeromus himself), as well as a variety of other offensive magic.
Zeromus appears in a non-corporeal form at the start of the fight, and will not attack and cannot be damaged. This is a good time to cast beneficial status effects. After Cecil uses the Crystal, Zeromus starts off with Big Bang. Rosa should use Curaja and revive any KO'd party members. Cecil should attack or, alternatively, use Elixirs, while Kainjumps. Rydia can use her summons, preferably Bahamut. Black Magic and Holy should be avoided, as they will be countered by Flare. Edge can attack or throwFuma Shurikens or other powerful weapons, as they will not be needed when the battle is done.
When Zeromus begins to shake, it means he is about to perform Big Bang. Rosa should keep steady with the healing while the others unleash what they have against the boss.
In the DS, mobile and Steam versions, Zeromus will counterattack Rydia's summons with Osmose, and Black Magic and Holy with Flare. Zeromus will shake when he gets a turn, dispelling all status effects on the party. A Dark Matter can be stolen from Zeromus. It can be used to fight the superbossProto-Babil.
Using summons prompts Zeromus to counterattack with Osmose, but this can be exploited; if used at the right time, Rydia's summon will cause a slight delay and counter with Osmose. However, this will not prevent Zeromus from using Big Bang immediately after.
Rosa should heal while the others concentrate in using their best attacks.
Phase 1 (start of battle): Disrupt, Disrupt, Big Bang, switch to phase 2.
Phase 2 (regular script): Disrupt, Disrupt, Big Bang, Black Hole, Disrupt, Disrupt, Big Bang, Bio, Black Hole, Disrupt, Disrupt, Big Bang, Black Hole (repeat until either phases 3 or 4 start).
Phase 3 (see below): Disrupt, Disrupt, Big Bang, Black Hole, Flare, Disrupt, Disrupt, Disrupt, Big Bang (repeat until phase 4 starts).
Phase 4 (uses when below 12,000 HP and phase 3 was activated beforehand): Random action, Meteor (repeat until dead).
"Disrupt" (SNES translation) is indicated by Zeromus shaking. Zeromus absorbs Flare, presumably to halt what would otherwise be the most efficient method of dealing magical damage.
If Zeromus has a turn while he is between 16,000-12,000 HP and in phase 2, he will refill his health to the max (this action is invisible to the player) and switch to phase 3. He will not do this if the party knocks his health below 12,000 HP before he can heal himself, thereby leaving him in phase 2.
In the Easy Type version, Zeromus appears with a different battle sprite for its third battle. While this sprite is only used in this version, the design is later reused in some later versions for the Zeromus EG enemy.