Hailstone is an enemy ability used by Ultros during the battle at Esper Caves, Yeti, and Landworm. It reduces one target's HP by 1/4 its current total at the cost of 20 MP, and is unblockable but fails on targets immune to Instant Death.
Hail is a form of solid precipitation. It is distinct from sleet, though the two are often confused for one another. It consists of balls or irregular lumps of ice, each of which is called a hailstone.