stunned by boss
Stun is part of TERA's combat system. Stun is few seconds unconsciousness that prevents player or monster using any skills, attack or defense themself. Stunned player or monster has rotating stars around their head.
Stuns happen due dungeon special attacks or in PvP due attacks by other player. Using iframe when attack happens prevents getting stunned.
Stuns make certain classes, like Warrior, powerful in PvP, since warrior's Backstab also stuns player.
In PvP a knockdown on stunned player cancels the stun and they can escape with retaliate. It's bad idea to use any skills that might knockdown a stunned player. Rather use stuns skills to continue stun duration.
There are also certain skills that raise stun resistance and prevent you getting stunned in PVP. Priest can prevent stuns from players with Guardian Sanctuary. With other classes that have such skill it's now generally merged to Tenacity. These skills don't prevent stuns from dungeon special attacks.
Mystic can escape stuns while casting Corruption Ring. This skill also works on dungeon special attacks.
Stun duration can be reduced with Earrings rolls. Both earrings can have −10% stun duration line. Stun can also be Cleansed by healer classes from other players or with Cleansing or Quatrefoil brooch from themselves.