Uisang is purported to be the founder of many temples in Korea upon his return from T'ang China. He is the one who introduced the teachings of the Flower Garland Sutra to Korea and established the Hwaom Sect in the country. Pomosa is one of the ten great Hwaom temples set up by Uisang. It is not clear if he actually founded the temple or if it was already in existence and simply expanded by Uisang. There is no doubt, however, that it's association with Uisang and Hwaom philosophy was the main reason for the temple's growth during the Shilla and Koryo periods.
Here is a translation of Buddhapia.com's brief biography of Uisang.