((Kami is a powerful character, however, he has weaknesses. His powers require an energy source, he has a massive ego, his lightning doesn't affect things that conduct well, and insulators block his attack. His powers can be blocked. I have three rules of OP that determine whether something is too overpowered.
1) can it be avoiced? If it cannot, it is quite probably overpowered, unless it's really weak. Your power has this problem. Neither physical attacks nor energy attacks affect the locusts, instead making them stronger, meaning that literally nothing can be done to stop it. The only thing I can think of is someone else with Dark Magic trying to undo the spell, or maybe anti-magic. However, that's rare.
2) is it too large? An attack that wipes out countries is probably too powerful. Your attack violates this because it has the potential to wipe out the entire universe. As it grows with energy, it can feed off of the energy of everything. Pretty much everything is energy. Thus, unless stopped, it could easily engulfe the universe.
3) does it have too massive an effect on the character? An attack that is instantly lethal, or mind-control, should be used with caution. Attacks that have too large of an effect, are usually overpowered. Your attack violates this because it is irreversibly lethal to my character, first draining and then killing him.

Sometimes, these rules can be waived, if there is an excuse. For example, if your sole power was to destroy/absorb/corrupt electricity, then I'd be fine with it, as that is specifically your power. However, you have freeform magic. This means you can do pretty much anything you want, create any attack you want. Thus, you're incredibly adaptable, and have no real weaknesses.
For all the reasons above, I believe the Wormwood attack is overpowered. However, as the GM has not intervened, I cannot remove it, or prevent you from using it. I simply wished to state my case.))

Kami notices his power is depleting, and starts franticallly drawing in power. However, much of it is already corrupted, and burns him instead, as he shouts out in agony. He glares at Abbadon, as red lightning mingles with his yellow, and he tries to purify himself.