A commander with the assassin ability may attempt to assassinate a random enemy non-hidden commander in the same province. Assassination attempts occur before many other events in the turn order, such as movement. If the assassin is in a province where there is a hostile fort under siege, they must have the Scale Walls ability to be able to assassinate a commander inside.
If there is a non-hiding enemy commander in the province, an assassin will always find a target and trigger an assassination battle. There is a 50% chance that an assassination will occur at night, which has potential to incur some darkness penalties. Commanders that are assassinated are considered to be surprised and will disregard any scripted orders they are given, including conservative gem usage. The assassin however will carry out their script as normal.
A potential assassination victim can have bodyguards, which are units under their command with the Guard Commander order. Each bodyguard has a base 50% chance to be present in the assassination, which is increased by the Bodyguard ability.
Similarly, the value listed on the assassin ability is the patience score, which reduces the odds of any bodyguards being present at an assassination attempt.
A unit with the seduction trait can effectively serve as an assassin.