I'll touch lightly on some of the major quests and difficult areas in Rhun.
The "Epic" quests: There are 2 epic quests (according to the Rhun Modding Team). The first main quest sets you on the path to face a very powerful, ancient and evil being and his minions. It will take you across many areas of Rhun and other planar areas. 4-6 players level 40 and loaded for bear...in some areas your teleport items won't work, so you can't jump out and go restock on healing potions. Lord Vaude will set you on the path to this quest when you are of the proper level.
The second main quest (more properly "The Lady of the Moon's Quest") will take you across various areas of Rhun, but all paths lead to the dreaded Underdark and the domain of the Drow. 3-4 players level 38-40 (again loaded for bear, i.e. epic level armor/weapons and items). Guard Captain Siger (patrols in from of Kedylath Temple will set you on the path of this quest.
The quest to gain access to epic items is epic in itself.Can be done solo level 38-40 +8 weapon or better. The Taer Overseer in The Desert will inform you of the required steps.
The Duegar Caverns are an especially difficult area. Most of the Duegar weapons have "Magebuster (TM)" technology, so spells and buffs will not do you much good. Like many of the dangerous areas including the epic quests, It's been my experience that there is simply no substitute for high hit-points, a high armor class and a high attack bonus.
There is one more area I'll mention that should be ranked as epic...facing 3 Dracoliches at once, amongst other foes should be counted as epic
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It's the Taer Prison Isle Quest. It's very difficult to solve this quest, just finding your way down is a pain. I don't think there is a quest giver (It was the original main quest in Djerv Ter's World of Rhun mod.) You'll face an Undead Bard as well as some of his sub-bosses. Can be done solo, level 40 with epic items/weapons etc..if in a group once you find your way in and kill him, wait and let him respawn for each member. He has 2 awesome items, a +13 Longsword (The Dreamcatcher) and a book that allows any class to cast a variety of damage enhancements on their weapon.
In some of these quests, notably the 2 epic quests, you probably won't be able to do them with an evil alignment party leader. Having evil members in your party may cause a quest stage to fail since some quest stage NPCs are hostile to evils, Drow, Vampires and many of the sub-races.
One final bit of knowledge: Most of the boss monsters can only heal 3 times...so, get them down to near death that 4th time, and they will go down. So can be tagged by Devastating Critical, but some are immune to it.
Hope this info helps.
-Alaric