Hello,
I've had a Sharn-based Lord of Dust as a big bad evil guy for 15 or so levels now. The party is slowly mopping up varies layers of minions, cronies and lackeys and getting close to the BBEG.
However I have no clue how they could actually find / neutralize a millenia old immortal who manipulates power from behind the scenes.
Any thoughts on how they could find / pin down / neutralize a LoD? What kind of defenses would such an entity have?
See Keith's article for some general thoughts, I'm looking for more specific inspiration! 
http://keith-baker.com/dragonmarks-34-the-lords-of-dust/
No real ideas currently except that the rakshasa lets himself be taken down. He'll get resurrected anyway - outsiders work that way in Eberron, if he's of high enough level - and the PCs just fulfilled an essential part of the Prophecy to release the rakshasa's master. Now the PCs have to work out what the other elements of the Prophecy are, and prevent them. Say, make sure Aundair attacks Thaliost, or Riedra gets to built monoliths in Khorvaire. You'll have a new campaign arc, and the resurrected rakshasa can come back to taunt them. By speaking stone, of course!
Mechanically, if you don't want him to simply respawn, you will need to bind him in some fashion. Which probably requires a really big khyber shard. I am assuming here thay by "Lord of Dust", you mean one of the very powerful fiends who serve an Overlord. If you meant the Overlord itself, I think you probably need a coatl.
Actually, the answer is relatively simple: It's all about the Prophecy.
What I've said in a number of places is that you can't physically kill a powerful fiend permanently. Kill someone like the Wyrmbreaker and he'll respawn within a few days, good as new. Getting him out of the way permanently requires binding. But there's a middle ground... which is killing him in a way that will take him longer to return, or to kill him in a way that will wipe out his memories and personality even if the spiritual energy forms a new fiend. And that's where the Prophecy comes in. The Lords of Dust are the spawn of Khyber. They are PART of the Prophecy. So the Prophecy can give you keys to hurt them. This is how the Dragons and Couatl were able to gain some ground in their war long ago. Want to get rid of the Wyrmbreaker? The Prophecy might give you the chance to eliminate him for, let's say, a century. It might involve...
- A specific person killing him
- A specific location where he has to be defeated
- A specific time when he has to be defeated
- A specific weapon that has to be used
- A specific thing that has to be done to that weapon
For example, it could be that he can only be taken out of play if he's killed by a descendant of Tira Miron, wielding a weapon that contains at least some part of Kloinjer, Tira's sword; it has to be done while both Shavarath and Syrania are coterminous, which rarely happens; it has to occur at a manifest zone to Irian; and the weapon has to be infused with the essence of a particular Couatl that fought Durastoran in the first age. Hopefully, one of the PCs happens to BE a descendant of Tira, even if they don't know it now.
This gives an example of setting up a combat scenario where he could be killed, but you'd still have to FIND out about this path of the Prophecy; find a suitable zone; calculate the proper time; find the weapon and somehow contact and gain the aid of the couatl; and then somehow lure the fiend to the killing ground. Much of this might require aid from the Chamber, the Undying Court, or some other force that has a keen understanding of the Prophecy.
Of course, there's a simpler ally who would have a keen understanding of the Prophecy and both the means and motive to deal with your Lord of Dust... another Lord of Dust. The key to releasing Overlords is to push the Prophecy down particular paths. If defeating your Lord of Dust is the first step on the path that will ultimately release Bel Shalor, than the Wyrmbreaker would be happy to help you do it... because again, only an heir of Tira can, and that's you. Essentially, he NEEDS you to defeat this Lord of Dust to help HIS Overlord. Which means that once you defeat him, you'll need to worry about the NEW Lord of Dust, but hey, one problem at a time.
Three things allowed the Dragons and Couatl to defeat the Overlords before:
- The discovery of the Prophecy
- The sacrifice of the Couatl
- The fact that the Overlords didn't work together
While the Lords of Dust are a loose alliance, most put the needs of their Overlord ahead of everything else - and that's an important tool your players can use.
Thank you three very much, especially Keith for the fulsome answer!
I am particularly interested in brainstorming regarding the types of entities that could provide the info to oppose a LoD:
This gives an example of setting up a combat scenario where he could be killed, but you'd still have to FIND out about this path of the Prophecy; find a suitable zone; calculate the proper time; find the weapon and somehow contact and gain the aid of the couatl; and then somehow lure the fiend to the killing ground. Much of this might require aid from the Chamber, the Undying Court, or some other force that has a keen understanding of the Prophecy.
While the idea of having a Lord of Dust mix it up with another Lord of Dust is very cool, this will be a ~ten level plot arc and I've had several "frenemies" already in the campaign. I'd like to have some entity that A) is unambigously not "cosmicly evil" and B) that isn't fundamentally more powerful than the players and C) won't represent a loose end that the players feel they need to wrap up.
I think the Chamber would fail on B. Undying Court could work, as could the Church of the Silver Flame. I'm leaning a bit towards the Silver Flame as we are a Sharn based group, and there could be some good superficial conflicts with Thrane as the campaign progressed. I'm also thinking of having the last / one of the last Coatl on the scene as a mysterious "deep throat" type individual ( http://en.wikipedia.org/wiki/Deep_Throat_(Watergate) ).
I may also have a capital G Giant (tens of thousands of year old veteran of the conflict with Argonnessen) on the scene for some explication on the true nature of evil in Eberron.
I'm thinking that there would be three or so critical set pieces where the Lord of Dust needs to accomplish some task in support of the Prophecy and the players have the opportunity to frustrate that task and strike directly at the LoD himself, with varying degrees of "permanancy" depending on their research and preparation. Right now I'm contemplating using a heavily modified version of the E1-E3 modules where Orcus is replaced with an Overlord.
Any other thoughts on foils who would be Prophecy-savvy without overshadowing the PCs or being a "loose end" in the campaign (e.g. Dreaming Dark) would be greatly appreciated!
How about Tasker's Dream, the Draconic Prophesy think tank set up by House Sivis? Nothing is more ambiguously aligned than a dragonmarked house. Especially if there are ties between that house and the Trust (can't remember where I read that, but it could be true even if it isn't canon).
If you have access to the City of Stormreach 3.5 sourcebook, you can also check out some of the affiliations. One, or several (I forget), were all about searching Xen'Drik for a certain tome that contains a partial written account of the Prophecy, the Caldeyn (or something) Fragments. Any of them would make good foils for the PCs of equal strength, since they'd all be NPCs with PC class levels of whatever level you need them to be.
Thank you both for the ideas.
I went with Tasker's Dream, and have had initial contact with a member of House Sivis who said she's been monitoring all the messages that pass through the House, and that they are tracking certain activities that seem to coincide with certain rituals that have been performed on a repeated basis, as early as 40,000 years pre-game time.
I'll get ahold of City of Stormreach - that sounds like a good plot device. As I've noted elsewhere, I plan for my players to destroy another moon of Eberron, which will require that they go huntin in Xen'Drik for Giant magic, etc. and will provide a good basis for bumping into this other group...