[Echo of the End].
The ultimate weapon of the old era used to combat The Inexpressible, a piece of completely self-contained information, impossible to be perceived in any way, hence the black cube appeared to allow the "perception" of such information, becoming the container for [Echo of the End] and making it perceptible.
After Lorenzo had a detailed conversation with Merlin, they confirmed one thing: this weapon can only be used within the Well of Ascension, and only it can suppress The Inexpressible within the well.
According to the intelligence from the Secret Keeper, ancient humans not only imprisoned The Inexpressible but almost completely [Exiled] it from human consciousness. Unfortunately, in the end, they fell short.
Because of this, Lorenzo had a conjecture even he found hard to believe: perhaps ancient humans were just one step away. If only the final [Echo of the End] could be released, maybe The Inexpressible could be completely [Exiled].
