Runic is just a flag for spells. You can view/edit this in Lord J's FF3usME (I've never tried changing these, but the option is there). Most spells in the Magic menu have it set, and only a handful of monster spells have it.
For the lazy reference:
Meteor, Quake, W Wind, Merton, Warp, and Quick are the only player-spells that don't have it set.
No Lores have Runic set.
Megazerk, Blaster, Atomic Ray, Tek Laser, Diffuser, Mega Volt, Giga Volt, 50 Gs, N. Cross, Hyperdrive, Disaster, Shim Sham, Scar Beam, and Fire Wall are the enemy spells that are susceptible to Runic.
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The cosmetics of it are probably related to the fact that a spell animation happens significantly later than the actual calculations of it. That is to say, the result of an attack is determined a few seconds before the character ever steps forward and does the attack. Runic probably is not checked until a bit later. I'm not sure about this, and it could use some investigation.
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As for the spelling, adding a 'k' is actually a rule in English. Although it is a very obscure one, I presume the rule exists to keep the hard 'c' intact, as generally 'c' before 'i' or 'e' is soft. So, 'Runicker', 'Runickee', but 'Runicable'...
