Purge

A Purge is a combination of basic dispel and movement speed (and sometimes also attack speed) slow. In some cases, they also apply root, preventing units from moving and using certain abilities for a short duration.

Mechanics
The dispel purges apply are basic dispels and it is only applied once upon cast. It is not applied continuously.

The slow of a purge works different than a regular slow. All purges initially slow a unit's movement speed down by 100% (and attack speed by 100, if the source of the Purge slows attack speed). The slow gets gradually weaker over the Purge's duration at a certain rate. The rate and amount at which the purge gets weaker is determined by the update frequency. The amount of instances equal to the amount of the update frequency, while the amount of slow loss equals to the initial slow amount divided by the update frequency.

If a purge has an update frequency of 1, it slows for 100% for its whole duration. If it has a frequency of 2, it slows for 100% for 50% of its duration, and then 50% for its remaining duration. With an update frequency of 3, it slows for 100% for the first 33% of its duration, then 66.66% for the next 33% of its duration, and finally 33% for the remaining duration. This means the higher the update frequencyis, the smoother the intervals and slow loss get.

The stop which some of the purges apply, is a root-like effect and is applied together with the slow simultaniously. The slow ticks down regardless, so after the stop duration, the unit slowed for the remaining duration. Since it is like a root, the unit's movement speed is not influenced by it. It is not set to 0 during the stop. However, unlike roots, no True Sight is provided over the targets.