My confidence level isn't great, but are you:
A Black Hole / Singularity
Locked in an eternal struggle to impede the son,
Could be a play on words between son and sun. Our solar system is orbiting a black hole the center of the milky way - locked in an eternal struggle perhaps.
By natural law, I am the true heir apparent,
This is the one I struggle to apply the most. A black hole is a natural force - and perhaps the "true heir" as if the sun goes supernova it will reign supreme
I am voracious, but nothing will whet my appetite,
No matter how much a black hole sucks up, it's going to keep on sucking
Until the day comes to reign over the masses,
Black holes have a huge mass, particularly supermassive ones.
From the very center I watch, calm but deadly,
The center of a black hole is the singularity which is infinitely small but hugely dense. No one really knows what happens there.
Spinning my thoughts, poring over my plans of destruction,
Black holes spin and are certainly destructive planet killers
Do not sell me short, I'm a remorseless killer,
Not being sentient, a black hole would kill without remorse
But whether you fight against me or succumb meekly,
There is really nothing that could be done if you come across one...
Hale and hearty I remain, reborn time after time
The black hole is going to remain strong and healthy. Reborn time after time as stars collapse.