We are aware of players unable to sign into Riders of Icarus using the Steam application receiving the error (999). This is caused by Steam outages occurring on their end, which we have reported and Steam is currently looking into to resolve.
If you wish to play Icarus, you will need download the Nexon America Launcher which can be found here:
Please sign in with your Nexon America account with the email you have linked to your Steam profile. We will provide updates when the issue has been resolved. Thank you!