I like your ideas.
Just to elaborate some things.
*The power source of magic is these Leyline nodes all over the world where this "wells" of magic reside, or where magical energies are in higher concentration. ( Places of power I suppose.)
They are all connected via leyline through the air and earth. So if someone's closer to one of these nodes when they draw magical power, the power is stronger.
The villain is a high elf, and knows arcane magic but kills this specific dragon out of jealousy and spite. At some point of this process he corrupts his already existing Arcane magic and is able use necromancy. However he is the first and only High elf in the story to pervert Arcane magic in this way.
I'm still trying to figure out
a) What he did to cause this corruption?
b) How would killing a dragon aid in this corruption process?
All magic in the story comes at the cost of life force, and for Arcane magic users, its their own life force that is drained by using spells. If it's a big spell or one they are not familiar with, it could possible be fatal for them.