The Sigil of the Magus is an artifact found in Hub 5: Necropolis, in Hexen. It is one of the three items that open of the doors blocking the way to Dark Crucible. It is available after killing Menelkir in his tomb. It is comparable to a smaller version of the Bloodscourge.
In Deathkings of the Dark Citadel, the Sigil of the Magus makes a return in the Nave.