When the tremor subsides, a new passageway is quickly discovered behind the collapsed north wall of the keep's prison. It leads to a stairway descending downward into darkness.
In this conversion, a hidden portal to one of Karzoug's many vaults lies beneath the keep. None of the Black Arrows were aware of its existence, but the quake has revealed its entrance. Now the undead charged with protecting the vault wait for anyone brave enough to test the runelord's defenses.
This short side trek draws heavily upon Dungeon #169's
Hall of the Snake God for inspiration. The maps provided there are great for these two encounters, with a bit of alteration. The first map's entrance should be a staircase leading down, not an entrance from the outside, and the snake trap should be removed from the pit in the lower chamber (though the pit is still present, courtesy of the quake that reopened these chambers).
These encounters also make use of some of the monsters found in the published adventure's bestiary, converted to 4th Edition. They are both fairly straightforward.
For the upper chamber, I suggest the following encounter for a party of five adventurers:
Total: 3,250 xp (Level 12 Encounter)
For the lower chamber, I suggest the following encounter for a party of five adventurers:
Total: 3,550 xp (Level 12 Encounter)
Neither of these chambers contains any treasure, but at the rear of the lower chamber is a glowing yellow portal surrounded on the ground by two alternating sets of blue and green runes. A Perception check (DC 25) notes that some of the green runes are sitting above small cracks in the ground, likely caused by the tremor. An Arcana check (DC 25) determines that the portal's condition is tenuous; by the time the PCs can examine it, only two minutes remain before the portal collapses completely. Looking through the portal, the party can see a room beyond filled with treasure but without doors or windows of any sort. Within the vault on the other side of the portal is a massive pile of Thassilonian coin worth 8,500 gold pieces, as well as a shining gold breastplate bearing runic markings - purely ceremonial, worth 6,500 gold pieces (these are treasure parcels 5 and 7 from level 11 and parcel 6 from level 12). The vault also contains treasure parcels 3 and 4 from level 12, sitting neatly in the room's corners.
The party does not have long to retrieve everything, and may need to employ some quick thinking to move nearly 200 pounds of coinage out of the vault in two minutes. When the portal collapses, the vault's contents are lost and anyone still on the other side is trapped with no clear exit. The green set of runes on the ground fades and disappears, but the blue set remains. This is actually a permanent teleportation circle, and allows a party with access to the Linked Portal ritual to memorize the rune set in order to return to Fort Rannick's basement whenever they wish.
If the party successfully explores the secret chambers, they receive three victory points.