No characters should be lost in the merge, but it’s important to note that your characters will be limited by your total number of character slots available. Your total number of slots on Fey Forest will be equal to your highest number of slots from the three prior servers, but “overflow” characters will be inaccessible until you delete some of your characters to make some room. Assuming your roster wasn’t full on all three servers, however, you should be able to log in and start playing today; here’s hoping to a minimum of issues for affected players and characters alike.
TERA characters on the Highwatch, Valley of Titans, or Lake of Tears servers? The proper answer is “no.” Even if you did have characters on those servers, you don’t any more, as all three servers have been merged together to form the new Fey Forest server. That server should be available with the end of maintenance at 12:00 p.m. EDT, with all other servers going offline at 10:00 a.m. EDT for standard maintenance.
Source: Official Site