I actually love this weapon, it's very unique in itself.
I have a few concerns though...
Does it turn into the phoenix version of ash? I believe it should honestly, a weapon that turns into ash and then into a phoenix is incredibly cool.
Secondly, there is an issue with the fact that phoenix in it's upgraded form is less powerful, it is an intention of deisgn, but may not be a good thing here. You need to figure out exactly what you want to do there.
Lastly, if this does in fact turn into a phoenix eventually, then combustion isn't really the best name, this is minor, but should be thought over.
However if the first condition is false and this is a unique type of ash that is rebirthed into the sword again, then I feel this card loses some of it's value.