There is already a high level death spell that does very nearly what disintegrate does. It's called "Kill" and requires death archmage ability. Honestly I never try it though. It costs a lot of mana, but usually by that point in a game I already have quite a bit, so the high mana is not the issue with it. It takes 2 turns to get off the spell... To make matters worse, if the target resists (per the description anyways) there is zero effect. 

So, my suggestion to you is to just edit "Kill" to what you had in mind for disintegrate. As useless a spell as Kill is imo, I might do the same thing.

I like your idea about cone of cold too. Water magic is missing a reliable evocation spell, as the chaos spell's outcome is too unpredictable. 

But..but, it's just not as cool as cone of cold...

CoreSpells.xml

Find Fireball and make copy of it. Change the names and values to your spell. Use your effect instead of the one there. It might take a few tries. Go ahead and post your XML here if you get stuck. I can help later today. 

It would have to be a radius in front of the player. Firebreath is the closest we can get to a cone spell. That might actually be the right spell to copy.