The Vault of Eternity is a magical prison located deep beneath Castle Ehb, even lower than its infamous dungeons. The 10th Legion's battlemages bound the Seck within this prison for three centuries, using the power of the Staff of Stars to maintain the wards that kept them in stasis.
When Grand Mage Merik disappeared, and the Staff of Stars stolen, the magical wards faded, and the Seck were able to escape from the Vault of Eternity and terrorize Ehb once again, an event known as the Seck Resurgence. Eventually, a hero was able to enter the depths of the Vault, and defeat the Seck and their terrible leader, Gom.
The highest accessible point of the Vault is where the Seck breached into the castle dungeons; into a not-well-traveled auditorium behind the last prison cells. It is likely here where long past Grand Mages kept the ward running.
By the time Lady Montbarron liberated Castle Ehb, the Vault of Eternity was a gas-choked, inhospitable cavern flowing with hot magma and crawling with the Seck and their horrid monsters. Clad in the toughest of armor and equipped with the strongest weapons and arcane spells from the Chamber of Stars, they went down into the earth, past lakes and rivers of molten rock, destroying every Seck they encountered. The temperature rose the deeper they went (Fortunately for them; Zed, Merik, and Andiemus were masters of ice magic), and more and more horrendous monsters stood in their way, but were cut down. Strangely, ruins of columns were found, and immense twisting spiked pillars bedecked with giant skulls were churning above Gom's throne room. The closer the party went to Gom, darker and quieter grew the air around the bridges that led to the Seck lord. At the end of the Vault was an elevator; timed by magic lamps, that descended to Gom.