The En Passant is the artifact from the Well of Dreams arc, but it isn't another name for the Well.

The En Passant is the artifact from the Well of Dreams arc, but it isn't another name for the Well. The Squire explains the problem in Wonderland #34. The Terror can torment people through nightmares and isn't something they can reliably stop by simply attacking a solid body. The En Passant can suppress his powers and force him to remain substantial. Getting it has its own restriction. The remaining artifact is associated with the Lion, and the Squire says a chess piece is needed to remove it. That makes Calie's role important to the recovery rather than simply making this a job anyone could complete by finding the right location. By #35, Calie and Cheshire have retrieved it from the Valley of the Kings. Calie feels an electric pulse from it. The Lion survives the encounter and returns to the mausoleum. So before the artifact is even used on the Terror, the story has established its purpose, where it has been kept and a condition on recovering it.