Unilord was God in that Universe. This is Perez's run of alteration of Silver Surfer. Being the strongest towards the end of his series...during the Unilord saga, it was the top being of another universe. This being had constructs that he had created called Blackbodies that absorbed the soul of beings on planets that he had condemned. There was a Watcher from that had gone to this universe millenia ago, who in exchange for his counsel to UniLord, UniLord put one soul in "Dark Counsel" Blackbody. The Surfer was defeated by Dark Counsel's Blackbody and was absorbed. Long story short, Surfer was able to gain control control of the merged form with the Blackbody and went into combat with UniLord. During this fight, Surfer used the power of the Blackbody merged with his own to drain souls from the UniLord until they were equals. Surfer then used this new power to drain the UniLord of all the souls he had gained over the millenia and thus gained UniLord's power level thus became the entire universe/universal continuum.There are more points to the story than this, but I'm just trying to give you an idea of who UniLord Surfer Time again, Surfer has the ability to merge, intermingle, mix, blend adapt to foreign energy. He is a conduit.
The Unilord is the abstractual embodiment of a universal continuum, an abstract conceptual entity. Go read SS 111-122. It , with a thought can hurl planets move galaxies with a thought. It was dominion over creation and for those of us without a dictionary handy ... the Uni Lord is basically the god of this strange, alternative universe that the Surfer was summoned to, and went to in order to help the inhabitants. It was nearly omnipotent, but needed a Watcher to keep him updated about the future on distant events. He gave this Watcher a "soul" from his cache as payment, so it clearly pained the Uni Lord to ask the Watcher for help. Basically, it above the physical plane of exitence. That is why Surfer is unique. His Soul is cannot be tampered due to theory of Galactus tamperement. Surfer fought at a different approach to attain where he was and this was a being, who Surfer fought it's manifestation which was made up mostly of prayers and souls. By assimilating the souls he assimilated the UniLord’s consciousness and became the UniLord.