Slow is a form of Crowd Control introduced in Diablo II, usable by both players and monsters.
The general concept is limiting the target's Movement Speed for a short time. This is most useful to prevent the enemy from escaping their doom, or from chasing down a hero willing to run with their life. It can also be used to kite the enemy without leaving combat, giving the advantage and keeping the victim at safe range.
In-game[]
Diablo II[]
In Diablo II, Slow can happen due to Decrepify Curse, Clay Golem's slowing attack and items with "Slows Target By X%". Even monsters that are immune to Cold can be slowed. Cast rate is not affected by Slow.
Slow works as a simple "speed debuff". Some unique and set items have the "Slows Target By X% mod; and there are skills with this effect like Decrepify, Holy Freeze and Clay Golem's attack. Notable that Holy Freeze's slow is not a simple cold and cannot be resisted with items with the Cannot Be Frozen modifier and Cold Immune monsters are affected by it as any other monster, only the periodic (and very low) cold damage will be resisted. Items and skills using this effect stacks with a cap about 85%.
Diablo Immortal[]
Slow icon
Slow returns in Diablo Immortal and can used by the player and monsters. Visually, slow can be identified by the blue ring that appears at the victim's feet. It can also be inflicted on the player by moving over murky ponds found in Bilefen or acid pools.
Diablo III[]
In Diablo III, Slow always has a small chance to trigger on non-player targets, even if no equipped items have the chance to Slow on hit affix. Being immune to Cold damage has no effect on Slow. Slow is also common, and affects both players and monsters. For example, Spiders leave pools of web that slow the players stepping on them by 60%.
There are three types of Slow: 30%, 60% and 80%. They stack, but cannot go over 80% at any time. The monsters' CC resistance does not change Slow duration, but reduces its potency, to a minimum of 5%. Some effects, like Slow Time, reduce not only the enemy's movement speed, but the attack speed as well.
Elites also have innate 25% reduction to Slow effects on movement speed and 65% reduction to attack speed penalties. The reduces duration of control impairing effects affixes on players' items reduce the duration, but players cannot reduce the potency of Slow effects.
Slow and Chill count as single type of CC for the purpose of CC resistance, and most effects that require the target to be Slowed to trigger do so regardless of whether it is a Slow or Chill.
Immobilize can be considered the extreme variant of Slow, just as Freeze is an extreme degree of Chill. However, Slow and Immobilize effects do not stack, do not override each other, and are not interchangeable for all purposes.
Diablo IV[]
Slow returns in Diablo IV, as a Sorceress class mechanic, also used to a lesser degree by Rogue. Here, Chill counts as a form of Slow.