So the ROT-13 substitution cipher says that you replace each letter by the letter 13 letters further along in the alphabet. For the last 13 letters, you continue on by rotating back to the beginning of the alphabet, which is where the idea of the alphabet being in kind of a circle comes in.
For each letter in the original message, you substitute with the one below it from the following list:
Code:ABCDEFGHIJKLMNOPQRSTUVWXYZ
NOPQRSTUVWXYZABCDEFGHIJKLM
So to unscramble the message, you replace each letter with the letter that is above it in the above list. (Or the letter below it - it doesn't really matter with the ROT-13 cipher.)
As someone mentioned, you read the message top-to-bottom, then left-to-right (Chinese-style), instead of left-to-right, then top-to-bottom, like English usually is. So the message, with the deciphered version below it is:
Code:X_RYHQBEZBYGN_NZR_VQ_YYNYGNQBEGLENAAVN_URAXHHXVXBYNX_ZN_NDHN
K_ELUDORMOLTA_AME_ID_LLALTADORTYRANNIA_HENKUUKIKOLAK_MA_AQUA
Separating the words, this turns into a list of Altador Cup teams:
K
reludor, Molta
ra, Me
rid
ell, Altador, Tyrannia,
Shenkuu, Kiko Lak
e, and Ma
raqua, where the missing letters have been filled in in red.
This gives us the letters rrreser. Converting them back into the ciphered version, we get eeerfre and unscrambling gives us referee, which is "marginally related to the Altador Cup."