One of three daggers found in the Ritual Chamber, the sinusoidal dagger has been specially treated ritually over the course of many nights of the full moon. It is discussed in the text The Sacrificial Dagger (found in the same chamber as the Dagger) and is the only weapon capable of harming the Vagabond.
The Sacrificial Dagger describes the weapon thusly:
The knife with the sinusoidal blade that has been dipped seven times, on nights when the moon is full, in water that has been distilled a hundred times, will be laid aside, since it would send the Vagabond back into his own dimension.