Ranger – “Emerald” [5th campaign, 2nd mission]

There, by the end of the second week, you need to capture the demon castle in the upper right corner of the map; that's when you'll receive the quest about the obelisks. Then, transfer your troops by sea and quickly sail to the lower left corner to defeat Deleb. (Sail in a circle through the top of the map – there's no passage at the bottom.) After that, pick up your own troops and head through the dungeon to the central castle.