The Dagger Obelisk functions as a +5 dagger. It's primary function, however, is much different.

If the Dagger Obelisk is thrust into fertile ground up to its hilt, a rumbling manifests and the ground parts to make way for a green, spiraling, 30 ft. by 8 ft. plant stalk that pushes up out of the ground and stops in front of the character. A grotto near the front of the stalk appears to be a passageway. This is, in fact, a portal to the Lost City of the Elders. The Dagger Obelisk is consumed when it creates this portal.

The Dagger Obelisk can only be constructed using a ritual described on pages 121-200 of The Tome of the Black Heart.

This requires three months of preparation time and magical investigation and a 30,000 gp sacrifice to the deities Fharlanghn and Celestian.

The ritual provides the XP needed to create the dagger. The components required for the creation of the dagger include 1 lb. of hornblende, a teleport spell, an extended plant growth spell, and a sapphire worth at least 5,000 gp (the sapphire must also be at at least 95% flawless).