The .rar extension indicates this is a compressed archive. Depending on where you found it, it could be:
An archived version of the game client or assets. Obcidian.Legacy.rar
Because the game was an Early Access title from around 2017, community members often share RAR files to preserve "Legacy" versions or specific build iterations that may no longer be available on official storefronts like Steam. equipment upgrades at a blacksmith
It includes standard ARPG systems such as quest-driven progression, talent trees, equipment upgrades at a blacksmith, and socketed gems to enhance gear. Obcidian.Legacy.rar
Files meant to modify the game or associated tools.