Spell distribution - Which file?

Hello there,
which game file exactly tells the game, which spells a cleric or a wizard can cast (has access to in his spellbook)?

My problem is, that my cleric has an altered "Gate" spell which summons a Death Knight instead of a fiend.

I however want to install the atweaks pack which alters exactly that spell.

So I want to make a seperate version of the "Death Knight" spell so that I will be able to cast both spells when I have access to lvl 7 cleric spells.