Dia1

Type White Magic
Level 1
Effect Inflicts 20-80 damage on undead foes.
Power 40
Hit Rate 96%
Target All foes
Price 100 gil
Sold In Cornelia

Learned By

WM
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BM
Kn
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Ni
The Dia Spells all do decent damage to all Undead Monsters, and no damage to anyone else. Because they are so limited, they generally do a lot of damage—Dia1 does as much damage as Ice1, a level 2 Black Magic Spell, but does it to multiple targets at level 1. Don't believe the bestiary when it tells you Undead are "Weak" to Dia Spells, though—this just means they're affected by them at all. Dia's damage is actually non-elemental, which means no Undead is weak to it, but no Undead resists it, either.
Compare Dia1 to other Spells
Shops: Where Dia1 is sold – 1 shown
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Cornelia L1 Wh.Magic
Cure1 Shld1

Related

Dia2

Type White Magic
Level 3
Effect Inflicts 40-160 damage on undead foes.
Power 80
Hit Rate 96%
Target All foes
Price 1,500 gil
Sold In Elfheim
Cast By Items Light Axe

Learned By

WM
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BM
Kn
WW
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Ni
Dia2 is a nice Spell against Undead, almost as powerful as Fire2. Either Spell will clear out most hordes of Undead quite nicely. The only problem with Dia2, as with all Dia Spells, is that it shares a Spell level with the Cure Spells, so you may not want to cast it too much. Dia2 is usually worth it, though, especially considering the relatively minor healing of Cure2 and Heal1. As with all the Dia Spells, only a White Mage can cast it, though late in the game you will find not one but two Light Axes that let anyone cast Dia2 for free.

Dia3

Type White Magic
Level 5
Effect Inflicts 60-240 damage on undead foes.
Power 120
Hit Rate 96%
Target All foes
Price 8,000 gil
Sold In Melmond

Learned By

WM
RM
BM
Kn
WW
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Ni
Dia3 is a powerful Undead killer, and it is available shortly before the Undead-filled Terra Cavern. It is a very powerful Spell, but unfortunately shares its level with the Life1 Spell. Don't waste all your level 5 Spell slots killing Undead, but don't be afraid to use Dia3 in a pinch.

Dia4

Type White Magic
Level 7
Effect Inflicts 80-320 damage on undead foes.
Power 160
Hit Rate 107%
Target All foes
Price 45,000 gil
Sold In Gaia

Learned By

WM
RM
BM
Kn
WW
RW
BW
Ni
Dia4 dishes out a whole lot of damage—just as much as Holy, in fact—but the problem is that there just aren't that many high-HP Undead to use it on. It can be a fun Spell to really put the hurt on Undead, but it's never worth wasting a Spell slot that could be used for Cure4 instead. This is one Dia Spell you can safely pass on.

HARM

Sprite
Power 40 Hit % 96%
Type White Magic LV 1
Effect Deals 40-80 non-elemental damage to all Undead foes
Target All foes

Learned by

WM
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BM
Kn
WW
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Ni
Sold In Coneria (100 G)
The HARM Spells all do decent significant to all Undead Monsters, and no damage to anyone else. They make up for this limitation by causing a lot of damage—HARM does as much damage as ICE, a level 2 Black Magic Spell, but dealt to multiple targets. HARM's damage is actually non-elemental, which means no Undead is weak to it, but no Undead resists it, either.