Arc Stone

This simple, black piece of obsidion, roughly the size of an Orange is strange to the touch, it alternatives betwen feeling quite too cold for comfort, then suddenly too hot, and in betwen that, it's sometimes sparkles with static electricy. Tiny golden runes can be seen inscribed in what looks like the stone's underside, which is almost flat. 

When properly attuned, this stone allows a caster, using spells with a particular damage type to change the type of damage from said spell.

For example, with proper concentration, a fire ball could be made into a frost ball, or acid ball if the needs where present. A lightning bolt could be made into a bold of cold. 

For the purpose of this stone, all damage types are placed into categories, and sorted thereafter. In order to change the damage type to another within the same category, there is no cost, but to change the damage type to a a type within a different category the spell has to be upcast, and use a spell slot of a higher level. 

 Due to the difficulty of changing the spells like this, it requires a special check  for the first 3 times the spell is cast with a different damage type. 

The DC for this is 10 + the spell's level. And failure of the check results in the spell slot being consumed, but the spell unravelling into nothing. a critical failure of this check (1 followed by a confirmation with DC 10 + the spells level) might result in the spell blowing up in it's casters face. 

 After a spell has been altered and cast without error 3 times, it can be cast from that point on without any checks, as the spell is then considered mastered. 

In order to change a spells damage type into one from the Positive damage category from a different category, it has to be cast with a slot 2 levels higher. For the other categories, it is enough to upcast it 1 level when swapping to a different one. Changing damage type wIthin the same category does not require a higher spell slot used. 

Notes: caster with spell slots