Blade bound magus would be my first guess. The archetype seems modeled after him.
Bladebound Magus, primary archmage with dual path into champion. Invest as many mythic abilities as necessary to make the sword a malevolent legendary artifact.
You could probably get similar results building him as an eldritch knight, but the Bladebound Magus is Pathfinder's not remotely subtle homage to Elric.
Well, if you built him at high enough level, he gets to add wizard spells to his magus list, and can get planar binding, etc. that way.
Otherwise, you'd want to build him as an eldritch knight if you want him to be a 1-9 caster who's highly proficient at killing people with his artifact sword.
Edit: Bladebound magus gives you an intelligent magic weapon at L3, but with enough mythic abilities invested in a weapon you'll still get an intelligent weapon of great power.
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The simplest way?
Bladebound magus with Champion as a mythic path most likely.
Now if you want to take into account his herbs, the fact that the sword gave him more con, the pacts with demons etc...guess it would be a little more like this:
Alchemist (Vivisectionist) 3/Magus 10/Summoner 7 - Champion ??
Use universal path to pick up legendary item and just bump up his weapon to make it become a major artifact eventually.
Alternative:
Conjurer 5/Fighter 1/Eldritch Knight 10 - Champion ??, just put some skill points into alchemy to represent the herbs etc...
I think he's a fighter/magus in a campaign world where the use of the Incantations rules are the norm.