Riposte and Backstab executions are what we call “critical attacks” where your weapon’s critical stat modifier plays a role. A riposte can be performed on a target that has had an attack parried, or has been guard broken due to stamina being completely depleted while guarding with a shield. On the other hand, backstabs are relatively easy to land and can be performed on the target if they are attacked from behind. The best dagger for maximizing such damage has to be the standard Dagger, which carries the highest critical modifier stat in the game. Pop it in your right hand with a Hornet Ring and you’ll be one-shotting enemies on ripostes!