The Silver key by Gage Prentiss
A mysterious artifact that can open the way to the Dreamlands.
According to Jonathan L. Howard's "Johannes Cabal: The Fear Institute", Silver Key is "the only artefact that allowed physical rather than psychic entry into the Dreamlands, and the only artefact that could be used by absolutely anyone with the will to do so". Probably the Key can also transfer its bearer through space and time, as Lovecraft's "The Silver Key" suggests. Randolph Carter finds Silver Key in his childhood home, and this allows him to enter a cave and magically emerge again in the year 1883 as a child.
In "Through the Gates of the Silver Key" Lovecraft mentions that the Siler Key is of Hyperborean origin.