[Anyone who wants to scout can, just describe your intended path and anything special you are looking for. Tabby will join the scouts unless Zafron asks her to stick nearby.]
Tabby and two of the Mondoon brothers spread out, approaching the hill... [Tabby will start off in whatever direction is not covered by the Mondoons]
Elidyr Pafiel, a robed elven man with a shield on his back and short sword on his hip, watches Bryce set out. He follows at a leisurely pace, but his eyes constantly scan the mound. His face shows little expression, but his hand lays on what must be a mage's components pouch
[Feel free to post character descriptions in the Roster thread]
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Bish:
1. It is said there were many leads during the years that all fail. What was the difference about this tomb lead?
The expedition that initially came to this location was the only one that found anything worth reporting. They are the expedition referred to in the following line from the intro post:
"Expeditions were sent to ascertain the truth of these various leads, but to no avail.. until most recently..."
2. What do we know about demi-lich? We are very high levels so we might know stuff. What about Lore knowledge about this Acererak?
Specifically about Demi-Liches, nothing beyond what one might speculate based on knowledge of a Lich.
To my knowledge this (Tomb of Horrors Module) is the absolute FIRST instance of a Demi-Lich. In the spirit of that, no character entering the Tomb of Horrors would ever have direct contact with one, or know anyone who had.
Liches, however, are likely known to higher level adventurers. They are the result of powerful mages who eventually sought immortality through terrible rituals in order to become undead. It would likely be known to at least some of the high level adventurers here that their spirits reside in a Phylactery, which must be destroyed to permanently dispose of them. They have the typical immunities of undead, and are always extremely high level magic users.
Are there rumors of how he raised to power? Special or preferred methods of killing? Same "Lore" about this tomb... any clues about what to be prepared for?
Given the consideration above, all that can be gleaned in this regard is what was provided in the Intro post
3. Do we believe the tomb is the origin of the pestilence? Acererak is the origin of the pestilence?
Per the intro post, all of the magic employed to determine the cause/origin of the pestilence pointed to various leads. The only lead that panned out brought the party here.
"Shordenth... has spent no small amount of his vast fortune... to unearth cure or cause. Expeditions were sent to ascertain the truth of these various leads..."
Why would he create such pestilence specifically against IronGards clan?
No one living knows. You can ask Acererak that if/when you find him 
Are others immune to the pestilence?
only those in some way related to Shordenth have fallen ill. His clan is quite large and even those of the weakest ties by blood to him have been afflicted. There are thousands of them and they are of the royal line.
Do we think that killing him will cure the disease? Or that the cure is in the tomb?
That is obviously what Shordenth hopes for, but it has not been confirmed. Still the reward is dependent on ending the evil that caused it and says nothing about curing them.
And why would the demi-lich live in the tomb for so long all by himself? Isn't he bored? Does he build something or research something that need to be there specifically?
"Live" in the tomb isn't the wording I would use 
Once again, to ascertain his reasoning, you would have to ask Acererak 
4. You mentioned that the guides are "few survivors". Survivors of what? Of previous expedition?
Yes. Are they NPCs?
Four of them are among the camp trail men and expeditioners. If not, wouldn't they never want to come near again??
They did not want to come back, they wanted to stay home. but the King's orders and reward for taking the party are not something easily ignored.
Are they from the clan? What information they can spare besides the way to the tomb?
They have given all the info they can. (see intro post)
Have they met something inside the tomb?
They have not been inside
5. You also mentioned that large groups are required. Are we a large group?
I would say 4 PCs and 20 pregens is a large adventuring group, yes.
6. Does the swamp water flow? Is there a current that can give clue to underground entrance (or escape)?
The water is brackish, stagnant and still. It does not flow in any perceivable way. Nice question
7. Who has danger sense?!?
Lol. Pik did, but he died. Bryce has it, but that doesn't mean Galan knows that 
8. Is Zafron the scout?
Anyone who wants to go first can be but Bryce is technically the Rogue. 
