Mordenkainen's arcane seal

Mordenkainen's arcane seal was a powerful spell, presumably developed by Mordenkainen of Oerth, that warded a space against entry. "It's protected by the arcane seal of Mordenkainen."

- Sofina, not elaborating

Description
It was marked by three glowing red magic circles on the entrance to the warded area: one small, one large, and one small, arranged vertically.

Effects
Reputedly, the spell could make a vault impenetrable to entry by physical means and all but the most powerful magical means. It seemed possible for a portal, such as produced by a hither-thither staff, to bypass the seal, provided one end of the portal was moved physically inside the warded area.

The seal could be broken by powerful magic like Mordenkainen's disjunction.

History
In the late 1490s DR,, Sofina, the wizard advisor to Lord Forge Fitzwilliam of Neverwinter, cast Mordenkainen's arcane seal on the treasure vault of the refurbished Castle Never to protect the valuables that should be held within. Forge offered use of this vault to his wealthy guests from other cities, such as Din Caldwell and Porb Piiradost, who'd come to Neverwinter to bet big on the High Sun Games and had brought their riches with them. This was a problem for the thieves Edgin Darvis, Holga Kilgore, Simon Aumar, and Doric, who had to seek nothing less than the helmet of disjunction in order to break through it.

Ultimately, during the start of the Games, Simon used the helmet of disjunction to break the Mordenkainen's arcane seal and enter the vault. But it was all for naught—the treasure had already been relocated.