That's as far as you go... Beyond is where the power of the Void is contained. You shall not enter!
Twintania is a boss and one of the Demons of the Rift in Final Fantasy V guarding the pathway to the crystal zone atop the dimensional castle in the Interdimensional Rift. It has two forms: the regular form, and the second form that appears after it utters "Charging Gigaflare!"
Twintania will attack with Atomic Ray, Snowstorm, Wind Slash, Mind Blast, and physical attacks; he will also start charging Giga Flare, at which point he will become vulnerable to various statuses, including Petrify and Death. He will usually counter physical attacks with Tidal Wave and magical attacks with Mega Flare.
A simple strategy which will result in little to no damage to the party, as well as killing Twintania quick enough that it will not start charging Giga Flare, is to equip the party with Reflect Rings and the Black Mage class and have every character multitarget high tier elemental spells on the party. The spell will reflect on the boss four times. Twintania counters with Mega Flare, but it will also reflect.