Rebirth Online is a free fantasy action MMORPG from Caret Games that entered Steam Early Access in summer 2020. It offers an open 3D world with real-time combat built around large fights.
The main activities are hunting field bosses with other players, boss raids and guild-versus-guild battles out in the open world. PvP is not confined to arenas; you can run into rivals anywhere. Judging by the developers' own description, nearly everything is built for groups and guilds.
It targets fans of mass PvP in the style of Korean MMORPGs. Be cautious, though: the last Steam news appeared in March 2021, and it is unclear whether the servers still run. Check community activity before investing time.
