Kingslayer’s Critical Hit Damage Bonus Effect
Once Executioner is maxed out to increease the number of critical hits you cause, Kingslayer becomes the next best source of increased damage for your character. Now that you are criting more often, making those hits even stronger begins to really amplify your damage output.
The base damage bonus for critical hits, is an additional 50% of damage caused. With Kingslayer maxed out, this becomes a 75% damage bonus, which is a quite substantial increase.
How to Find the Kingslayer Trait
Kingslayer is unlocked by defeating the Undying King world boss in Rhom. You can only encounter him in Story Mode, in the final area of Rhom, when you open the sun temple. You can start the fight with him before or after traveling to Corsus, by denying him the heart (Refuse options).