He was Galactus' first creation, pre dating the Heralds by billions of years. This was back when Galactus only consumed a planet every century or so so a herald was not needed. Tyrant was made for the purpose of being Galactus' son and he technically is. Being less wise at the time , Galactus granted Tyrant power on par with his own. Tyrant betrayed Galactus and a massive battle ensued which Galactus barely won. He limited Tyrant's power after that and cast him out. But Tyrant's power gradually grew. Later on he managed to defeat a moderately weakened Galactus but was thwarted from killing him due to Galactus' most loyal herald Morg. Among his other feats he stalemated Thanos as the Avatar of Death, and easily beat Silver Surfer, Gladiator, Beta Ray Bill and Ganymede at the same time. Think of Tyrant as Galactus at 50-60%. For instance in his fight with Galactus, they destroyed multiple galaxies as a side effect.