Sedulous Icosahedron

The dungeon walls are mortared stone, damp and mossy. The ceiling hangs heavy with calcerous drips, lost roots and fronds of lichen. In the centre of the chamber’s north wall there is a round, ragged indent in the stonework, with stones lying scattered on the floor. A single, glossily clean, roundish boulder bulges from the indent like a pustule. Peppered with thousands of gemstones of all colours and laced with thick veins of gold, it is a treasure-hunter’s dream. And colossal! It stretches almost from floor to ceiling, a geometric marvel of sharp, triangular facets.

Approaching, you see some writing is carved on each facet – the one facing directly into the room says “START ME UP.” On the surrounding facets you read “HOW DOES IT FEEL?”, “PLEASED TO MEET YOU” and “IT’S JUST YOU AND NO ONE ELSE.” A single number 1 is visible, half buried in the wall.

If the players leave the stone alone, well, they will have left the dungeon’s single biggest treasure behind.
If they hack away at the wall and free it, it rolls onto the ground and totters a little. Instead of coming to rest on one face, its rocking motion increases until it starts rolling.

It will pursue anyone who has touched it until it crushes them. Essentially, it’s like the giant rock at the beginning of Raiders of the Lost Ark, except it never, ever gives up until it is destroyed.

diameter: roughly 9′
HP: in the thousands
It rolls initially at walking speed (4mph). For every 10′ it rolls without changing direction, it can increase speed by 5mph, up to 70mph on clear, flat terrain. It always knows where its target is. If they cross a sea, it will roll across the bottom to get to them. It is immune to fire, cold, poison (obviously) and lava damage – although if it passes through a long lava field, it will emerge red hot. If it runs into an object like a door that stops it, then it backs up as far as it can and rams it again. If 10 rams don’t knock the obstacle down, it looks for another way around it.

If it has multiple targets, it pursues the nearest one. So it is theoretically possible to trap it like Buridan’s Ass.