In Forbidden Instances, certain bosses have the ability to summon assassins, that is, other players, to attack the party.
When the boss is attacked, that boss turns invulnerable for a short while. At this point, other players across the server will receive a request notice. If they accept, they will be warped into the instance where they must defend the boss and kill the raiding party within 10 minutes.
The number of assassins that appears depends on the boss and instance, though there is a chance none will appear at all.
The assassin's strength level corresponds to the player who attacked the boss (i.e. a Realized Potential will summon other players who are Realized Potential). Thus, it is strongly recommended for the lowest leveled player to attack the boss first.
If the assassins manage to succeed in defending the boss, the assassins will disappear, and the boss gains a buff that generates a shockwave over a very large radius and deals massive damage, effectively wiping out the raiding party, until the boss resets.
Rewards are given regardless if the boss survives or not.