Cactuar King is an optional enemy in Final Fantasy X bred at the Area Conquest section of the Monster Arena. It is, in essence, a tougher version of a normal Cactuar. It becomes available after capturing at least one of each fiend at the Thunder Plains, and the prize for unlocking it is 99 Chocobo Wings. Defeating it will net some Blessed Gems.
Cactuar King is immune to all five elements. It will counter all attacks with 10,000 Needles, targeting a random party member, meaning only those with the Break HP Limit ability can withstand it. On its turn, the King may unleash 99,999 Needles (50% of the time), which cannot be blocked and is always fatal. Alternatively, it may use 10,000 Needles (33% of the time), or simply do nothing. Once it has done nothing twice, it will instead have a chance of escaping. Unlike Qactuar and Cactuar, its remaining HP has no influence on the chance of it escaping. If the Cactuar King escapes, the player will not receive any rewards.
Cactuar King is extremely evasive; the Accuracy stat has no impact on the chance of connecting, which is always 25% before Luck is taken into account. Thus, the remaining 75% must be made up with the Luck stat, or use of Aim, Luck or Jinx. It is often simpler to use attacks that cannot miss; while Cactuar King's Magic Defense is too high for magic to be practical, Overdrives work well.
Although not as effective as Don Tonberry, Cactuar King is also well-suited for the AP trick. By equipping weapons with Overdrive → AP, and optionally some combination of Double AP, Triple AP, SOS Overdrive, Double Overdrive or Triple Overdrive, and using Stoic or Comrade Overdrive Mode, it is possible to gain sphere levels very quickly.