of gods and monsters- Tom Riddle

Both had dark mind and both had plans.
Obsessions and lust filling their hearts
Magic was a wonderful thing and so was its darkness.
And when you're beyond magic's capabilities and beyond power of nature that's when lust of power grows and grows until it drowns you.
Thus, they had no other weakness than each other, so they made sure they were powerful enough not to get killed.
For one must live, for the other to survive