There are seven collectibles you can acquire from killing enemies and for completing quests in the Yuida Adventurer's Tome in Lost Ark, and one of them is called Oath of Bandits
Oath of Bandits can only be obtained from the continent of Yudia. This collectible is not tied to a singular enemy. Rather, they drop at random from all enemies in that region.
Our Oath of Bandits dropped from Bolchi, and Whitesand Carrier/Scout. The drop rate is exceptionally low, and you'll only get a handful from completing quests in Yudia, but you will get nowhere near the amount needed.
If our theory is correct, farming a single enemy is pointless. Advance as you would, through the main quest, and massacre all enemies you come across and repeat dungeons.
You need to collect 25 Oath of Bandits for those looking to complete the Adventurer's Tome fully. Once you have found one, right-click it in your inventory, and it will automatically be added to the tome.