Here’s an adventure! 

While the heroes are traveling through a narrow valley, they spot an evil keep. The massive fortress of dark stone is 35 feet across and nearly 50 feet tall. Flags hanging from iron spikes depict flames, gruesome beheadings, and other wickedness. Two massive iron ballistae are mounted atop the keep’s tall towers. 

Creature: If the heroes approach, the evil keep attacks!

The Evil Keep      CR 19

XP 204,800
Advanced fiendish mimic
NE Colossal aberration (shapechanger)
Init +3; Senses darkvision 60 ft.; Perception +38
AC 19, touch 1, flat-footed 19 (-1 Dex, +18 natural, -8 size)
hp 445 (27d8+324)
Fort +21, Ref +10, Will +19
DR 10/good; Immune acid; Resist cold 15, fire 15; SR 24
Speed 10 ft.
Melee slam +35 (8d6+33/19-20 plus adhesive)
Melee slam with Power Attack and Greater Vital Strike +29 (24d6+45/19-20 plus adhesive)
Ranged masterwork heavy ballista +12 (4d8/19-20)
Space 30 ft.; Reach 30 ft.
Special Attacks constrict (6d6+33), smite good (+0 to hit, +27 to damage)
During Combat The Evil Keep slams with its Power Attack and Greater Vital Strike feats as often as possible. It only fires its ballistae when foes remain out of reach.
Morale The Evil Keep has existed for centuries pretending to be an isolated, evil keep to lure heroes to their deaths and then moving on. It’s never lost a battle, so it fights until destroyed.
Str 54, Dex 8, Con 34, Int 10, Wis 14, Cha 10
Base Atk +20; CMB +50 (+52 bull rush); CMD 59 (61 vs. bull rush, can't be tripped)
Feats Awesome Blow, Exotic Weapon Proficiency (ballista, heavy), Greater Vital Strike, Improved Bull Rush, Improved Critical (slam), Improved Initiative, Improved Natural Attack (slam), Improved Vital Strike, Iron Will, Lightning Reflexes, Power Attack, Skill Focus (Perception), Vital Strike, Weapon Focus (slam)
Skills Climb +32, Disguise +30 (+50 when mimicking objects), Intimidate +30, Knowledge (dungeoneering) +10, Perception +38, Sense Motive +15; Racial Modifiers +20 Disguise when mimicking objects
Languages Common
SQ mimic object
Gear masterwork heavy ballistae (2) with 10 ballista bolts

Treasure: When the Evil Keep is destroyed, it disgorges its offspring: a small egg untainted by its parent’s evil. The Junior Keep is an intelligent instant fortress with empathy, senses to 60 ft., and darkvision. It’s alignment is Neutral, and it has an Intelligence of 10, Wisdom of 12, and Charisma of 10. Its Ego is 8. It can cast disguise self three times per day, but only to appear as a different kind of building.

The End