Trolloid Geriatric Petrification

Trolls are not native to the prime material plane, but those who do find their way here tend to wreck havoc. They are hardy and are able to regenerate severed body parts over time. They are also wily enough to know when not to attack and when to retreat. The most potent limit to their impact on the folks of the land is the fact that on the material plane eventually, gradually but ultimately trolls turn to stone.

Causes

Trolloid Geriatric Petrification affects all Trolls who dwell on the prime material plane. It seems to be simply an aspect of their genetics. Onset appears to be linked to time spent on the plane, regardless of what age the troll was when they left the Shadowfell.

Symptoms

The first symptom of Trolloid Geriatric Petrification (estimated at about 80 years) is a stiffening and slowing of the joints. It is possible that trolls feel pain and soreness ahead of these effects. Since trolls don't speak common and generally don't try to communicate with the people of the prime material plane (only eat them), this can't be confirmed.
This is followed by the hardening of the skin (at about 90 years). By this stage the stiffening and slowing means that the troll is less likely to attack people passing by and much less able to catch them if they flee. But attacking the troll now becomes more difficult. It is still able to harm people, if it can catch them, but piercing it's hide is much harder.
In the final stage all movement stops and the troll turns to stone. It is suspected that sunlight plays a significant role at this point. A troll can survive many years at the tough-hide-stiffened-joints stage by remaining underground during the day.

Treatment

The recommended treatment of a troll experiencing TGP depends upon the stage which they are at.
Stiffened Joints Only When a troll has been terrorising your town for eighty years, look for signs that movement is slowed or full range of movement isn't being achieved. If this is seen, there is a window of ten years when this condition will impede their ability to attack or defend, but before the toughened skin provides them with extra protection. The appropriate treatment is with a sword or axe, applied to the vital organs, along with fire and acid if possible. Use your new-found superior dexterity to avoid retaliation and treat the troll as it deserves.
Stiffened Joints and Toughened Hide After the condition has progressed, the added toughness of the hide can make the treatment recommended for the previous stage much less effective. At this point it is better to treat the troll with blunt force violence. A sledghammer applied to the forehead can be successful at resolving the issue.
Ready for Petrification After another ten years have passed, or the troll stops appearing during daytime, it can be assumed that the troll has entered the final phase of TGP. At this stage, it is best to keep away from it at night but track it during the day. If it's hideaway can be identified then consider traps and burial as appropriate ways to treat the troll. A reverse trap can be effective too, setting up a mechanism that will block the entrance when the troll leaves at nightfall. This treatment is only effective if there is a lack of alternative hiding places, leaving the troll exposed to the sun when dawn comes.