To break the Oath of the Ancients in Baldur's Gate 3 and unlock the Oathbreaker Paladin subclass, you need to commit acts that violate the oath's core values of protecting life, nature, and the natural order. Specifically, you can break this oath by performing any of the following actions:

• Kill an innocent, non-hostile NPC.
• Kill the Owlbear and her cub in the Owlbear Cave.
• Free the imprisoned goblin Sazza in the Druid Grove by helping her escape.
• Kill the Tiefling Pandima at the Druid Grove.
• Use Auntie Ethel's wand to bring Mayrina's husband back from the dead (resurrecting the dead is considered a violation).
• Attack the Tieflings surrounding Lae'zel early in the game (this breaks any Paladin oath, including Ancients)

After breaking your oath, you will experience a vision and be visited by the Oathbreaker Knight at your camp, who offers to initiate you as an Oathbreaker. Accepting this will unlock the Oathbreaker subclass, granting you new dark powers that replace some of your Paladin abilities

. Note that breaking the oath often involves morally questionable or evil acts, such as killing innocents or harming nature unnecessarily, which contrasts with the oath's original ideals