Started on this concept...
Weapon will be replaced by a cooler, needlessly large weapon.
I know...too small. Squint.
Larger concepts will come soon.
Edit: Thinking about some sort of half machine, half creature... where the machine half has stopped functioning, so its become more dependent on the biological half (like a person losing his sight). The machine side is rusting out and being filled with plant life (not to mention drags behind the creature... maybe it could be enchanted and used as a weapon); the bio side is ancient--after all it was made a thousand years ago. But it's still a strong combatant because of the magic it learned throughout the ages.
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Here's that cyborg idea...
Used to be a warlord, now an invoker. Not sure about the artifact.
Edit: Scrapped. Goin' with the mysterious void creature.
Quick paintover of the thumbnail and added a few details. Badassitude +/-?
Still deciding on an artifact. Was thinking a black hole would be pretty neat to chain and drag around...or part of the void he came from.
I am a fan of your idea so far. I think a lot of people will miss the post technological aspect of the theme, and go for straight fantasy. I think the fact that you are keeping that as a big part of your concept will make for a strong and original design.
The black hole idea seems a little impractical though. I liked the original one that seemed like a burden he has to drag along with him. Perhaps combine the two ideas, keep the box, but have the negative energy emanate from it...so it's kind of like a pandora's box idea.
I think you could really benefit from an asymmetrical design here too, perhaps a larger arm to drag the weapon, or have one side bursting open with the vegetation overgrowth.
Look forward to seeing where you go with this!