Reuben took the tableland east of the Dead Sea, in the country that Moab also claimed, and the biblical record of the tribe is short and mostly critical.
In the wilderness narratives, Dathan and Abiram, who join Korah in challenging Moses, are Reubenites, and the ground opens under their households. In the Song of Deborah, one of the oldest passages in the Hebrew Bible, the tribes that failed to come to the muster are named and mocked, and Reuben gets two full lines: great searchings of heart, and a question about why he stayed among the sheepfolds to hear the pipings of the flocks. Moses' blessing in Deuteronomy manages only let Reuben live and not die, and let not his men be few, which is faint even by the standards of that chapter.
Then the record thins out. Reuben produced no judge, no king and no prophet. The Moabite inscription of King Mesha, which describes his campaigns over exactly this ground and names the men of Gad by name, does not mention Reuben at all. That is an argument from silence and should be treated as one, but it fits the pattern of a tribe that was small, exposed, and probably absorbed by its neighbours early.
Genealogies in Chronicles keep the name alive and record Reubenite clans and their pasturage. The tribe survives as an entry in a list, which for a firstborn is the precise shape of the sentence pronounced over him at the beginning.