It originally appeared as an enemy ability used by Yunalesca from Final Fantasy X, being a more powerful version of the Death spell. However, it has since also become an signature ability for Ardyn Izunia from Final Fantasy XV.
Mega Death appears as an enemy ability in the raid dungeon Weeping City of Mhach. Used by Forgall, Mega Death inflicts Instant Death raid-wide to all members of the expedition. To avoid Instant Death, a character must be under the effect of Gradual Zombification, obtained by getting hit (intentionally) by one green floating bubble summoned by Evil Mist. In addition, they may also avoid the attack's effect if they are accidentally zombified from gaining three stacks of Gradual Zombification.
Megadeath is a term for one million human deaths, usually caused by a nuclear explosion. The term was used by scientists and thinkers who strategized likely outcomes of all-out nuclear warfare.
Mega is a unit prefix in the metric system denoting a factor of one million (106 or 1,000,000). It has the symbol M.