HP to one ability

HP To One abilities are a recurring theme in the Final Fantasy series. They reduce the HP of one or more party members to one, regardless of their current HP. This is sometimes done by simply setting the targets to one HP, at other times by inflicting damage to them equal to their max HP minus one. They are commonly used by Final Bosses and Superbosses. One of the most well-known of these abilities is Heartless Angel, which has become the trademark attack of Sephiroth.

Final Fantasy VIII
Omega Weapon has a similar ability called Megiddo Flame. The ability does not reduce the party to one HP, but deals a guaranteed 9,998 damage to the entire party, and thus the only way to survive is to have 9,999 HP, leaving the party with one HP after the attack hits.

Final Fantasy IX
Beatrix uses Stock Break and Climhazzard to reduce the party's HP to 1 when fought as a boss, and these abilities cannot be blocked or avoided. This leads to the end of battle once she has performed them. When these abilities are used by the player's party, Stock Break and Climhazzard instead simply deal major non-elemental damage to all enemies.

Final Fantasy Dimensions
Shango uses Divine Storm to reduce the party's HP to 1 when fought for the first time. This leads to the end of battle once he has used it. The same goes for Glacial Dance used by Cocytus. The battle are scripted events when these are used. When these abilities are later used by them during their real battle, they deal normal damage.