Dear Milletians,
It has come to our attention that there is a bug related to the latest Windows 10 Internet Explorer Flash Player update which is causing the game to crash when launching from Steam. While we are investigating the issue there is a verified work around which will get you back into the game. If you uninstall or do not update to “Security Update for Internet Explorer Flash Player for Windows 10 Version 1511 for x64-based Systems (KB3132372)†the game will launch normally. This issue only appears to impact users of Windows 10 so all other users can continue to play as normal.
Thank you for your patience while we investigate this issue further.
Source: http://mabinogi.nexon.net/Community/forums#%2Fshowthread.php%3F1511114-Windows-10-Steam-Issue%26nxid%3D10
Alternatively if you don't want to uninstall a security update you could just direct launch from your steam folder which I'm hoping everyone here knows how to do that. If not, you can typically find it in this folder: C:/Program Files(x86)/steam/steamapps/common/Mabinogi and then just launch Mabinogi.exe like you would normally if you weren't using steam.