From Descent-Community Wiki 1.1
Add 1 additional green power die to your attack pool.
If you are defeated, place the sun stone token in your space.
|Q: When the Sun Stone is equipped by a hero, can that hero perform an attack and elect not to use it, or is its ability to add the green die compulsory?|
|A: The card as written does not give a choice, so the added green die is compulsory.|
|Source Kara Centell-Dunk, Game Developer|
|Q: What happens if a hero with the sun stone is defeated and the sun stone token is not picked up before the end of the encounter?|
|A: The sun stone is returned to that hero.|
|Source FAQ v1.6 page 7|