Like all +5 weapons, the DM will add +5 to the AC hit when using this weapon but the damage bonus has already been factored into the damage wheel.
When the wielder of this weapon scores a natural 20 with an attack, all attacks made by the victim suffer a -2 damage penalty. A second attack with this weapon which scores a natural 20 will reduce the victim’s melee damage by an additional 2 points, but -4 is the maximum penalty this weapon is capable of inflicting. It doesn’t matter how many Wicked Spears the party possesses or who used them.
The damage penalty effect of this weapon is triggered by sliding a natural 20.
The rapier will fight by itself as long as Widseth is within 30′. Physical barriers do not sever the mental connection between the rapier and its owner. It can’t chase and fight flying creatures, but it can be extended (within the 30′ range) to fight a foe separated by a chasm or physical barriers. While the sword fights, Widseth may do whatever he likes, including attack with another weapon, cast spells, or perform bardsong.