Who and when united the Volyn and Galician kingdoms into one Galicia-Volyn land?

In 1199, Roman Mstislavovich, with the help of a squad and some townspeople, occupied Galich and united the Galicia and Volyn principality. During his short reign, he fought not only to keep the new principality from collapse, but also seized the Kiev region, Pereyaslav region and Chernigov lands. After his death, there was a temporary decay of these lands. However, in 1238, during the struggle with the Golden Horde, the son of Roman Mstislavich Daniel managed to restore the unity of the principality, which existed until 1349.