This champion cannot be Stunned, Slowed, Ensnared, Paralyzed or Rooted, and is immune to Knockback. This unit is also immune to any effects that cause the loss of AP.
Unstoppable champions can not posess.
Unstoppable prevents many of the ways to control a champion, but note it doesn't prevent everything. Unstoppable champions can still be Dumbfounded, Antagonized and so on. It also does not exempt the champion from spending AP on attacks, movement or anything else requiring spending AP. (In other words, AP lost is different from AP spent.)
Also, it is important to note that if you gain Unstoppable while affected by one of the conditions that it prevents, you will not lose the condition. For example, if you equip a Rage Band on a Stunned champion, the champion will remain Stunned.