Describe the problem. Be as specific as possible!:
The area boss Leviathan used to spawn in the map Leviathan's Canyon, which is a hidden street off Wyvern Canyon, in Leafre.
With the introduction of the old (JMS) Silent Crusade, it got put into a Master Monster Portal on that same map.
At the switch to the new (KMS) Silent Crusade, the Master Monster Portal was removed, but Leviathan was not returned to spawning freely in Leviathan's Canyon.
This is a bigger problem than it seems, since Leviathan is part of the Leafre Set of the Codex, which is one of the best sets and highly sought-after.