Head to the cemetery and resolve the issue between the ghosts and see the quest giver in Clever Clogs tavern and tell him it was just a marital spat.
Speak with the hermit there and he will tell you how to obtain the sword from the lake by proving the 5 virtues of Honor, Wisdom, Compassion, Generosity, and Valor.
You will head towards Lac Celavy Lake, east of Corvo Bianco. First you must complete the Quest There Can Be Only One, which can be picked up at a notice board.
Aerondight is a Silver Sword in the Witcher 3: Wild Hunt Blood and Wine.
