Old School RPG (i.e. Second Edition) Dungeon

When within 5 spaces of the obelisk, it increases max stat by 3. If max stat is tied, roll to randomly decide which one is increased.

Every 2 turns the obelisk modifies current alignment one step toward Lawful Good. If already Lawful Good nothing happens.

If within 5 spaces of the obelisk for 5 or more turns, acquire 1 roll toward a Secondary Skill and gain 1 additional slot toward a non-weapon proficiency.

If the Players stay within 10 spaces of the obelisk for 24 hours the effects are permanent. If not, the effects fade away over the next 24 hours.

Touching the obelisk triggers a self protection spell of a Lightning Bolt, Fireball, or Ice Storm based on a caster level that should not kill the Player but provide a stern warning. If a Player touches the gem they appear to die instantly. What happens is their soul is thrown 40,000 years into the future and then returned exactly 40,000 years from when they touched the stone. Generally, as their body has most likely turned into dust long ago, they roam the land in that form forever.

Obelisk Powered Keep

A keep materializes seemingly out of thin air in a field near the Players.
If they approach and Elder Elemental greets them at the open portcullis. It seems the previous inhabitants of the keep have long since passed away and there is still work to be done.
One of the Players, if "worthy" will be the new leader of the keep. At this point the DM can decide what tests are required to be completed, but once finished the Elder Elemental will turn over an extremely detailed key that opens the Obdurium walled room in the basement that houses the obelisk.
The obelisk powers the keep, allowing it to shift as per the plane shift spell and appears 5-500 (5d%) from the intended direction. However, the stronghold can fly and is also submersible.

Once found worthy, the Elder Elemental will assist the Players and will also provide direction to the next adventure.

The general layout of the keep:
Obelisk Room

Fist Floor:
Common Area
Servants' Quarters
Dining Hall

Second Floor:
Guard Towers
Bedroom Suites
Study / Magic Research