Hex spells are the counterpoint to augment spells, weakening the enemy and making them easier to slay. Hex spells can only ever be cast on enemy units. Hexes can be cast onto units in close combat. Targets of hex spells need not lie in the the caster's forward arc.
Note that bonuses and penalties from Hex, Augment and other spells are cumulative, but normally cannot take any characteristics above 10 or below 1.