The Nexus offered Toby a way to reclaim his human lifestyle, and the promise of immortality. But nothing ever goes entirely to plan, and the enemies of the Nexus are many. The Sentinel is forced to find a balance between his new allegiance to the machines and his old bonds of friendship and love. But the greatest betrayal comes from within, and Toby might find himself looking entirely in the wrong direction when the hammer falls.

Author's Note:
The Fourth Age part of the Secret Histories story is detailed in the Pact Arcanum Saga.
For readers who don't want to dive into over 1,800 pages of additional material, this series of novellas directly reprints selected portions of the Pact Arcanum Saga which are relevant to the final book of the Secret Histories: The Fifth Age.
As such, readers of the Pact Arcanum Saga may skip this section without compromising the overall narrative.