vivi_ultima is correct with L3 Flare, although that was his second guess.
Regardless, you're up vivi.
The explanation is provided below.
A fiery nightmare / Refers to the fact that L3 Flare is fire-elemental
for those who see threes / Only characters whose levels are multiples of three are affected
Able even to bring
heroes down to their knees. / Should a character/s have a level divisible by three, and have no fire protection, the spell will reduce his/her/their HP to 0. The formula for L3 Flare does damage equal to 5/4 * HP, so the technique is always lethal. Yeah.
Unspeakable power / Same as above, obviously.
Though subject to chance / L3 Flare is used as a counterattack, and only randomly
And only magic demands / Only magic can elicit this particular counter
this most horrible glance. A reference to the Allemagne, the only monster in FFVII that can use L3 Flare. Allemagne is a mistranslation of Ahriman, well known in the series as a flying eyeball-type creature (hence the glance).
