In the wastes of Lower Druk Yul, where only the occasional grasshopperman tribe wanders by but otherwise the wind is left to its own devices, there are kurgans - burial mounds - made from the dessicated husks of huge, long dead tortoises. Only the shells remain, hollowed out, along with the contents which ancient people put there.
There are a dozen of these kurgans dotted across an area of 100 or so square miles. They are so far from civilization that they are largely unknown and untouched. Each contains TT: B, along with something or somebody dead. Breaking into any of the tombs brings a curse on the head of the first interloper to enter. Slowly but surely he or she is transformed into a tortoise; the process takes 6 weeks and cannot be lifted except by a remove curse spell. This halts the effects but does not reverse them - the curse can only be completely cured by a wish spell. Mechanical effects are as follows and are cumulative until the transformation is complete:
First week: skin toughens and back hardens (+1 bonus to AC)
Second week: back curves and hunches (+1 bonus to AC, -2 DEX, -2 CHA)
Third week: mouth forms into beak (gets an extra bite attack doing d3 damage)
Fourth week: back hunches further and hardens into a carapace (+1 AC, -2 to hit)
Fifth week: limbs stiffen and standing becomes difficult (+3 damage from strength, -2 to hit)
Sixth week: character becomes a tortoise, without the ability to function in human society - though he or she retains sentience.