Well Mannered is a hidden achievement/trophy in Dishonored.
How to Acquire[]
To acquire this achievement/trophy, Corvo Attano must locate and eliminate the true Lady Boyle without disrupting the party or causing any disturbances whatsoever.
Corvo has two options: the lethal approach, killing her and disposing of her body, or the nonlethal option, knocking her unconscious and delivering her to a noble named Lord Brisby. Either method can be chosen, but Corvo must not disturb the peace throughout the duration of the party.