This attack slows the target until it ends a turn. Slow halves the damage caused by attacks and the movement cost for a slowed unit is doubled. A unit that is slowed will feature a snail icon in its sidebar information when it is selected.
The Slow ability is extremely powerful because it has such wide-ranging effects. Enemy undead get half the normal benefit from their Drain ability, the damage bonus from Charge or Backstab is cancelled, and the Berserk ability is weakened. Fast foes struggle to escape; slow enemies are almost immobilized, and your own units suffer less retaliation damage, so it’s quicker to heal them. 
Combining multiple special abilities can be deadly. While it may take some work to combine, for example, Leadership, Illuminates and Charge against a slowed opponent, the results can be impressive. ~High Lord Kalenz, 470 YW~