"When a charging enemy suddenly feels a tremendous weight on his back, he can only slow his pace to compensate even if there is nothing there to explain this unusual heaviness."Slow is a second-level order magic spell in Heroes of Might and Magic IV that costs three spell points and causes the enemy target to move at half speed and movement.
While it should be obvious that keeping your enemy's melee creatures from swiftly advancing on you by stealing away their movement rate is highly desirable, of equal importance is the reduction they will suffer in combat speed, giving your creatures and heroes the increased likelihood of going first during a round. Casting this spell on an enemy target virtually assures that, no matter their range, they are highly unlikely to be able to reach your formation on the first round of combat.
Unlike with Fatigue, Slow works on virtually any creature - the target need not be alive to be affected.
However, Slow can be negated by Haste and Speed. Artifacts such as the Ring of Speed will also mitigate its effectiveness, while Mullich's Helm of Leadership, the Helm of Command, and movement bonuses conferred by heroes possessing the Tactics skill tree can provide speed and initiative bonuses to help keep Slowed creatures moving.