Slow Movement is an affliction; a negative status effect that handicaps the target.
It is also known as Snare.
A creature under the effect of Slow Movement will simply move slower than usual while under the effect. The degree to which they are slowed varies based on what kind of Slow Movement is used.
Players are able to inflict Slow Movement on a target using the Slow Movement spell from the Offensive Affliction Magic skill. This spell is first available at level 12 magic. Like with all affliction spells, a link must be maintained to keep the target slowed. This slows movement speed by about 20%.
Players can increase their chances of resisting a Slow Movement Affliction spell by wearing jewelry that raises their Forest resistance.
Another way to inflict Slow Movement on a target is by inflicting a critical hit on the target's legs with either a melee or a ranged weapon. This Slow Movement lasts a few seconds for every hit.
The Slow Movement caused by a critical hit to the legs is not a magical effect and thus Forest resistance will not help resist it. Creatures that are immune to Slow Movement spells similarly are not immune to this effect.
Since homin mages are capable of casting Slow Movement affliction spells, it may be used against you in PvP, though it's not very popular there.
All creatures using melee or ranged attacks have a small chance to land a critical hit to your legs and thus inflict a short Slow Movement.
Najabs can inflict yet another type of Slow Movement on their enemies with their spit attack. This also is not a magical effect and thus able to bypass both Forest resistance and Slow Movement immunity.
The following creatures are immune to the Slow Movement spell: Arana, Arma, Capryni, Gubani, Igara, Jugula, Messab, Najab, Ocyx, Raspal, Torbak, Varinx, Yelk, Kincher, Kipesta, Kizarak, Frahar.
Slow Movement immunity only applies to the Offensive Affliction spell; not to Slow Movement caused by critical hits or the Najab special ability.