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

Effects
Reputedly, the spell could make a vault impenetrable to entry by physical means and all but the most powerful magical means. It was marked by three glowing red magic circles on the door to the warded area, one small, one large, and one small, arranged vertically.

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.