Tyrant is a powerful supervillain from the Marvel Universe, and is an enemy of Galactus, his heralds, and cosmic heroes and villains. It has been stated that if Galactus is a god then Tyrant would be his devil. In many ways, that is how the two characters related to one another.
Many aeons ago, when the universe was still fairly young, Galactus created an immensely powerful living machine empowered with his own cosmic-power and, in many ways, fashioned on himself. However Tyrant became a power-hungry monster that sought conquest and ruin - unlike Galactus, who wished to serve a higher purpose.
Eventually Galactus learned of his creations ambitions and fought him in a titanic battle that was said to be sufficiently extreme it tore apart galaxies - ultimately Tyrant lost the battle and was stripped of much his cosmic power then banished to the furthest reaches of the universe by Galactus.
Thousands of years later Tyrant would return, having gathered a vast arsenal of weaponry during his exile. He begins kidnapping cosmic heroes and villains in order to harness their combined power; he uses to begin to fulfill his dreams of conquest. Among the more famous of Tyrant's many victims were: The Silver Surfer, Terrax, Morg, Beta Ray Bill, Gladiator, Jack of Hearts and Ganymede. Before Jack of Hearts was captured, he activated a fail-safe within his suit, destroying the containment unit and freeing Tyrant's prisoners. They proceed to fight Tyrant but he easily defeats them all. Soon after Galactus himself arrived, searching for Morg, his current Herald, but Tyrant would not release him. Instead, he offered to Galactus all his other captives in exchange for Morg. Despite great reluctance Galactus accepted the offer, not wanting to battle Tyrant at that time.
Battle With Thanos
Later, the mad Titan known as Thanos is seeking a challenge and learns of Tyrant. He allies himself with Terrax, Ganymede, Jack of Hearts and Captain Marvel to learn about Tyrant but abandons them to gain access to Tyrant's computers. The heroes fight Tyrant again but he defeats them. After the fight, Thanos threatened to steal an orb containing a part of Tyrant's energy and challenges Tyrant to stop him.
Thanos was able to stand his own against Tyrant during the fight, although it caused great damage to Tyrant's fortress and forced the heroes to flee, Thanos was able to teleport away and take the power orb with him.
Some time after his battle with Thanos, Tyrant revived Morg and inserted implanted within him; allowing him to mentally control Morg. He sends him back to Galactus as a trick; however, Morg manages to fight against Tyrant's control and obtains The Ultimate Nullifier. In the end, Morg would use the Ultimate Nullifier against Tyrant to kill him.
Tyrant was amongst the most powerful of Marvel's supervillains, capable of taking on Galactus himself. Much like Galactus, Tyrant is dependent on an outward source of energy in order to keep himself alive. While Galactus gains his energy from planets Tyrant gains his from superhuman beings, using them as living batteries.