Here is an adventure that I ran for my group when they were about 4th - 6th level. I offer it for free (and very unpolished!) through this blog post. Please don't repackage it or try to sell it.
You can find the location of the Palace on the campaign map here:
The Palace Vulre has been lost for several thousand years.
It was built in the vicinity where Sanctuary now stands, somewhere in the Misthall Mountains by Fieryat Lys before Ormolu was even founded.
The palace was constructed of exquisite materials: opalized wood, jet & amber.
It is themed after Lithlan: the Cunning Sign, the mighty hero and demi-god of the Hjolk-trull.
Hjolk-trull Legend states the place was buried in mud when a volcano erupted and formed the Valley of Dekkon-Tharr.
Occasionally, Hjolk-trull from the far north have come in search of the lost palace, but none to date have ever found it.
Deep in the crags above Sanctuary, on the northern face of the mighty snow-packed mountain known as Geir Loe, there is a fissure that offers shelter from the raging blizzard.
Beneath the snow is a hole, a break in the palace dome that drops 70' into an icy but unfrozen pool.
Odds are that one of the party will fall through this void and into the palace, thus discovering it.
The Treasury Riddle
Strange fruit wakes in Esma,
Spoiled in a bygone day.
How many fruit there glisten
And swell with un-decay?
The number known to Lithlan:
More than a hundred twice
Yet well below a thousand
Were sunk beneath the ice.
Lithlan asks the count and
He waits for you to speak.
A single error sends you
Without a chance to shriek...
[Each time the treasury riddle is answered wrong, the person who answers is level drained 1 level and teleported in a random direction at a distance of 1d100 miles.]
Good-size palm-filling stone of amber. Smooth and rounded but carved with the Cunning Sign.
If the stone is rubbed vigorously for 1 round against clothing, it generates a charge that lasts until discharged.
The stone will discharge if touched to another creature. It can also be hurled 10', 20', 30' range and will discharge on impact but has a 1in3 chance of breaking each time it is thrown.
The plate armor +3 is gold and bears an inscription in Old Speech reading "the castle"
VTT Player Map: