Alterna is a Ring Magic, obtained during Chapter 13 at Zegnautus Keep. Alterna is the last spell Noctis learns on the Ring of the Lucii. It casts a spell that expels an entire horde of enemies to the void at the cost of spending all MP. There is no EXP penalty for defeating enemies this way.
When the ring is in the weapon slot, the player can cast the Alterna if there is enough room in the field for the animation to play (it will not activate in narrow spaces) with the warp button when locking onto an enemy. A crystal sphere appears around the target(s), which collapses upon itself, sucking the enemies into a black hole that disappears. Very few enemies are immune to Alterna (such as the Dread Behemoth and Tyraneant), but the chance of it working is random and Noctis's Magic stat doesn't appear to affect it. It works against Gladiolus Amicitia, Ignis Scientia and Prompto Argentum when they are fought against at training session at camp. Casting Alterna costs a full MP bar.
Patch 1.06 boosted Alterna, making it work on more enemies, and even have a chance to work against Adamantoise. If Ifrit is defeated by Alterna, it triggers the death animation where Shiva defeats him. Patch 1.08 was released with a patch note saying that it fixes a bug that arises when using Alterna on Ifrit.
Alterna is a Level 8 Black Magic spell, only usable while equipped with the Ring of the Lucii. It inflicts one hit of massive non-elemental magic damage to all enemies (5.1x) and ignores 25% of the target's SPR stat at the cost of 80 MP (originally 99 MP).
Alterna is a Latin verb, a first-person singular present active imperative of alternō. From the word alternus, meaning "one after the other" or "alternate".