The complete history of Tenebrous is unknown. He has existed in this universe since its infancy. He is a proemial being. It is said that he prepared what existed for what is to come, by eliminating early lifeforms such as the Chaos Mites. It is documented that ages ago he was known and feared as Tenebrous of the Darkness. Time has erased nearly all of his evil. In an early battle of the Gods Tenebrous and the female proemial God Aegis were among the only remaining survivors of a brutal battle with Galactus. They were imprisoned in the prison planetoid known as the Kyln by the World Devourer Galactus. The Kyln is located at the edge of the universe in a sector known as the Crunch Cascade. They remained locked in there for centuries.
Unfortunately it was through the Crunch Cascade that the Annihilation Wave started its invasion of the positive universe. The wave did irreparable damage to the Kyln, and Aegis and Tenebrous were released back into the living universe once more, adding further fuel to the chaos that was engulfing all life. There were two other former powerful inmates freed from the Kyln, Kosmos the Maker formerly the Beyonder, and the Fallen One first Herald of Galactus. Tenebrous captured and drained the Fallen One of his power, leaving him as a lifeless husk. Thanos of Titan came to the rescue of the Fallen One. Thanos brokered a deal with Tenebrous for the return of the Fallen One.
Tenebrous and Aegis confronted Galactus and his newly restored Herald the Silver Surfer in battle. Galactus and the Silver Surfer were easily defeated and left for Thanos to strip them of the Power Cosmic. Thanos who had struck an alliance with Annihilus of the Negative Zone. Thanos worked diligently to remove the Power Cosmic. Tenebrous and Aegis disappeared into the cosmos. They later returned to the ruins of the Kyln hoping to find the other Proemial Gods, which could have also been prisoners there. To their disappointment they found the fallen body of Antiphon the Overseer. He had apparently fell victim to the destruction of the Kyln. The Silver Surfer who had escaped captivity tracked down the two Gods. He defeated them by using the powers of the Crunch to destroy them once and for all.