Cyclone is a druid ability learned at level 78, very similar in mechanics to Banish. Targets affected by Cyclone cannot move, perform any action, are immune to receiving heals or buffs, but completely immune to all incoming damage and unable to be debuffed. Cyclone is affected by diminishing returns, both in PvE and in PvP, but it cannot be dispelled, except for Priest's Mass Dispel.
You can only cyclone one target at a time. Cycloning a second target will cancel the one currently in effect.
Due to diminishing returns, you will have three successful casts on a single target before immunity begins. This spell's duration is cut in half every time, beginning with 6 seconds, dropping to 3 on the next use, and finally 1.5 seconds.
Cyclone can be tricky to use because in addition to the affected target not being able to perform any actions, no actions may be performed upon the affected target. A cyclone at the wrong time can cause another player to waste a long cooldown or a high cost spell, whereas a cyclone at the right time may cause a player to stop casting a long or highly damage attack, or even cause them to not receive a large heal that was coming their way.
Cyclone cannot be broken by abilities such as Divine Shield or Ice Block, and cyclone cannot be cast on players already affected by these types of abilities.
Using cyclone as a long term form of crowd control in an instance can be tricky and difficult for the druid to manage, especially if the druid is the healer. In most situations it is more effective to use Entangling Roots if it is necessary to crowd control on a large pull, or continuously throughout an instance. If the target that must be controlled is a ranged attacker however, then cyclone becomes especially useful.
When using this spell in situations where you're with others, grief can be avoided by communicating how long you will be cycloning, and when it has gone onto diminishing returns.