When the Grendel symbiote was excavated by S.H.I.E.L.D., samples of its living abyss were extracted and bonded to soldiers wounded in the Vietnam War, forming a black-ops initiative named the Sym-Soldier Program.
One of these offshoot symbiotes - codenamed "Tyrannosaurus" - was bonded to Rex Strickland. Enraged by the experiments S.H.I.E.L.D. was performing on the symbiote-dragon in order to find a way to kill it, the Tyrannosaurus symbiote - under the influence of Knull, the dark god who had created the symbiotes - seized control of its host and tried to corrupt him. For four mouths, Tyrannosaurus and its brethren prowled the jungles of Vietnam devouring any soldier they came across - Việt Cộng and American alike - until a Nick Fury Life-Model Decoy and Logan were dispatched to subdue them. The Tyrannosaurus symbiote separated from Rex and disguised itself as him in an attempt to catch Fury and Logan off-guard, but was exposed by Rex and briefly repelled. Upon returning with its brethren, it bonded to Logan but was quickly separated from him by Rex. When the LMD began counting down to self-destruct, the Tyrannosaurus symbiote - touched by Logan's compassion and influenced by Rex's nobility and inherent goodness - broke free from Knull's influence and bonded to him one last time, conveying regret for its actions. The Tyrannosaurus symbiote threw Logan to safety and tried to shield Rex from the ensuing blast, but failed.
After the death of its host, the Tyrannosaurus symbiote emulated Rex's appearance and personality to assume his identity; becoming an agent of S.H.I.E.L.D. ironically assigned with hunting itself down.
