Looks simple enough, everything looks good enough, but I need a few things better detailed before this can be approved
1. You say the sword glows with 'dark matter'. Is this purely for aesthetic? Or does it have a direct effect on the power of the sword.
2. What exactly is the 'toxic effect' the sword has. i.e. how does the toxin work/how powerful is it.
3. Define light matter. Without further definition, we cannot properly utillize this weakness. My interpretation so far is that 'light matter' is simply light magic. Additionally, the statement 'weak to light matter' suggests that it can be destroyed by whatever light matter is, which is something I don't imagine you want happening.