Thokol was the originator of a plot to infiltrate the Federation's Starfleet; after learning of a young Romulan archaeologist named Selke who held the katra of a deceased Vulcan named T'Priell in her head, Thokol contrived to have Selke press-ganged into the Tal'Shiar, surgically altered to resemble T'Priell and brainwashed into believing that she actually was the young Vulcan (her Romulan personality being completely suppressed).
In this state, she enrolled in Starfleet Academy as a member of Omega Squad. The plan was for her to rise through the ranks of Starfleet, with no one-- not even herself-- realizing that her loyalties were actually to the Romulan Star Empire; the ultimate sleeper agent. At one point, Thokol even intervened to keep Selke from being executed.(Star Trek: Starfleet Academy comic #12, Renegades)
The plan went awry, however, when, after months of having the two personalities co-habitating in Selke's body, the operative went insane, defecting to the Romulans much earlier than planned as Selke reasserted her control, taking the cadets of Omega Squadron prisoner.
When Nog went to rescue his classmates, a violent struggle ensued between Omega Squad and Thokol, during which Selke turned against him.
The Sub-Commander barely managed to escape from the confrontation, having been driven insane by the betrayal if his "creation" (Selke). He was last seen alone in a tomb in which was housed the corpse of the original T'Priell. It is unclear what became of him afterwards. (Star Trek: Starfleet Academy comic #14, Betrayal, #15, Origins, #16, The Fall)