Being the only offspring of Emperor Arcturus Mengsk, Valerian is next in line to rule the Dominion. He was conceived while Arcturus was on military leave and raised in hiding by his mother Juliana Pasteur until she was gradually sinking into a terminal illness. He was drawn to fossils and archeology, therefor studying literature around the mystic. He was considered weak by his father, more of a mother's boy, and having nothing in common. Arcturus fruitlessly tried to teach Valerian marksmanship, though they did have a common ground in form of fencing.
While Arcturus' spoils as a prospector made the family incredibly wealthy, the name Mengsk would come to haunt Valerian as he, and anyone around him, were subjects to assassination attempts more than once. It was not till after Arcturus had formed the Terran Dominion that he would get to see his father, a meeting ending in yet another assassination attempt. After the death of his mother, Valerian was moved to Korhal to secretly be raised as the heir of the Dominion, receiving teachings in leadership. Though unlike his father, Valerian believed in ruling without an iron fist and secretly looked up to Jim Raynor's honor and courage, being fully aware of his father's means to rise to power.
Having access to military equipment and mercenaries, Valerian continued his interest in archeology, studying Protoss artifacts and the mystery behind the Xel'Naga.
Hidden behind the Moebius Foundation and using Tychus Findlay, Valerian convinces Jim Raynor to acquire a series of Xel'Naga relics. When combined, they would become a mean to rescue Kerrigan, although this was only revealed to Jim after most the relic pieces were collected. Up to that point, credits was the only known agreement and Valerian's ties to the Foundation was unknown. Teaming up, Valerian declares to Arcturus that he intends to do what it takes to earn his place as the heir of the Dominion throne, and what Arcturus himself could never have accomplished.
Together with Jim's rebels, Valerian orders a full front assault on Char, using a massive Dominion fleet at his disposal.