The Antediluvian Vault is a series of underground caves and catacombs located beneath the Champion Academy in Aleroth during the events of Divinity II: Flames of Vengeance. The Dragon Knight can enter the vault after solving a puzzle in the Champion Academy to unlock the door. The vault acts as a prison and tomb for the essence of the mage Behrlihn.
It's unknown whether the Antediluvian Vault was constructed specifically by the Council to hold Behrlihn or if the ruined structures already existed and were simply repurposed as a tomb.
The vault is guarded by a several layers of magical security, but due to the warding spell being removed by Zandalor so he could use it to protect the city from the Black Ring, the Dragon Knight only needs to disable the remaining two layers; an illusory wall that requires a special helmet to see through and a puzzle involving the correct placement of orbs.
The Dragon Knight navigates these obstacles and enters the Antediluvian Vault late in the main quest in order to find Behrlihn, but is confronted by Bellegar who attempts to stop them for fear of what Behrlihn would do once freed.
Upon entering the Vault, the Dragon Knight finds Rhode who has been turned to stone. Bellegar quickly introduces himself, telling the Dragon Knight that he was the one who turned Rhode to stone for trying to find Behrlihn. He then summons a number of minions to kill the Dragon Knight, but once they are defeated he retreats into the vault. Bellegar cannot be damaged, and only retreats after enough summons are killed, and the remaining vault is full of the same types of enemies, all around level 40.
Down the corridor to the south, the Dragon Knight can enter two hallways to either sides, but eventually must continue to another split to the east and west. They both turn south and lead to their own crossroads, with an exit to the east. south of both crossroads are rooms, the westernmost one full of urns and the easternmost one a smithy with several low level weapons that can be picked up.
At the eastern exit, the cave curves northward to another eastward cave and a northern one that leads to a dead end. The eastern cave leads to a downward staircase lit with blue torches, eventually terminating in an ornate vault door leading to where Behrlihn is imprisoned. Bellegar appears one last time, reiterating that Behrlihn's release would destroy Rivellon, and offers the Dragon Knight the power to defeat the Black Ring without freeing Behrlihn. The Dragon Knight can either accept Bellegar's offer and leave Behrlihn imprisoned or free Behrlihn.