Stone statue, approximately one meter tall, depicting one of the Great Old Ones, Ghatanotoa. It was made of stone brought by Mi-Go from Yuggoth and was broken into six pieces.
Each part resembles fossilized parts of mixed bodies and bones of cephalopods, arachnids and fishes. Although it does not have any particular powers, it seems to be rather disturbing. Additionally, Detect Magic spell (or some spell or device with similar effects) may detect strong magical power sleeping in the fragments.
Dreadful truth about the statue is fact that anyone who look at the Ghatanotoa is instantly petrified. What is even worse - although victim turns to stone, it's still alive and conscious. Without possibility to hear, speak, see or move, each day he / she falls deeper and deeper into madness.
Of course, statue is an almost perfect image of the Great Old One, so it has exact effect as a god himself. No saving throw is allowed to overcome the effect and only spells like Wish or Raise Dead (remember that there is no "typical" Raise Dead spell in my campaign) or divine intervention can reverse the effect of the figurine.
Referee may scatter the fragments of the statue on the whole map of his current campaign or even place it in several places (such as treasure chests or monster lairs) in single (mega)dungeon. The choice is yours :D