There are magicks that messes with everyone's head - literally.
When caught, nothing happens (not even the slightest stomach ache).
But somewhere in the world, there's a headless being out to get you. Its head - or rather, the schematics for recreating the shape and functions and most memories - have been transferred to your DNA.
It rests there, inside you, doing nothing, until the poor headless creature comes within close range of you and the schematics activates - and you start transforming into its head. The creature will then pick up and attach its head (you), resuming business as usual.
Some explaining mechanics if you want/need that
Flow of the curse:
1. Person A is cursed
2. Random creature B loses its head
3. Creature B will start chasing after Person A (it will always know the general direction of Person A)
4. When Creature B is within range X of Person A, thing Y will happen (transformation still reversible)
5. When Creature B is within very close range of Person A, the head transformation will lock in and will complete
6. Creature B pick up its head and attaches it again
7. If Creature B is decapitated, the curse is reversed and Person A will unfold to its former self
To randomly determine what type of creature loses its head: use any random encounter table/long list of monsters you've got at hand. It may very well be another human being. Maybe not a kraken.
To determine how far away the creature is: common sense, or 1d12 days of travelling by foot.
Ranges and their effects (point 4 and 5 above): when Creature B is within 10 metres, the transformation kicks in and will not be reversible. Other than that: assuming a person has two legs, two arms, one torso and one head, render one random body part useless for every 100 meter the creature closes in (reversible), starting at a 600 metre range.
The Matryoshka Effect: however highly unlikely, there will always be a small risk that Person A, while cursed, loses his/her own head due to another person being cursed, and so on, ad infinitum.