Both games have a Steam page (here’s Aion’s and here’s Lineage II’s), with a nebulous “FALL 2015” listed as the release date. They’re the first two NCSoft games to make an appearance on Valve’s distribution platform.
Though there’s no official press release we can find, we do have a quote from NCSoft West VP of Commerce Jeff Pabst that’s making the rounds:
“Bringing Lineage II and Aion to Steam was a natural next step for both titles to further grow their prominence in the West. Both Lineage II and Aion have dedicated fan-bases, and we’re excited to work with Steam to enable even more players to experience these heritage titles.”
Both games have been available in North America for quite a while (as evidenced by Pabst calling them “heritage titles”), so it’s uncertain how much good a Steam release will do them, but it can’t hurt. Think you’ll be more interested in these two games once they’re on Steam?