The Digger's Guild (the PCs organization in the campaign) are hired to retrieve an artifact by a "mysterious benefactor." An earthquake has made an opening to lower Sharn and the PCs have a limited amount of time to explore the area before authorities find it and close it off (apparently standard operating procedure in Sharn).
When the PCs get there they find a group of House Tarkannen people camped outside the opening. They can attack or interact with the group.
When the PCs enter the opening things fade shortly thereafter and they find themselves in the bottom of a tower. What they don't initially know is they are in the bodies of people with Aberrant Marks during the War of the Mark. The tower is being attack by the comined fury of the houses.
You give the PCs story object cards that describe their dragonmark, it's ability, and a word that repeats in their mind (for example, the card that gives you "jump" has "twitch" repeating in their mind).
The PCs fight their way to the top of the tower encounter various obstacles, mostly from the dragonmarked house, including the Glyphweaver and Vadalis Bloodswarm. They also encounter an aberrant that drives in the fact that aberrant marks can be curses. They meet an aberrant who has been driven mad by his mark.
Finally they reach the top of the tower where a defense is in progress. Eventually they meet the Dreambreaker (a gnome with a powerful aberrant mark). If they touch his focus (the Delirium Stone) during the encounter, they return to the present and find they are in the room with the Delirium Stone, which is important in future adventures (the person who hired them is a descendant of the Dreammaker, apparently with the same aberrant mark).
They don't give the dreambreakers marks ability except when enhanced by the Delirium Stone (but do note it is far more powerful than any known today).
The aberrant mark (enhanced by the stone) allows him to:
Cast Confusion at will.
Cast Insanity at will.
Gives him a mental blast.
Uses Cha to modify hit points rather than Con.
"Sense Thoughts" gives him blindsense in a 60' radius, but only to detect living, sentient creatures.
"Shattering Touch" has the combined effects of Phantasmal Killer and Feeblemind.
If the Dreambreaker is ever killed, the Delirium Stone destroys his mind.
"Tower of the Mind" creates a mental repulsion that gives him a deflection bonus to AC equal to his CHA bonus (+7 in his case).
Additional undefined powers (i.e. plot elements).