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But until that moment on the boat, did Harry have any clue that Outsiders were Nemesis? Or why does everyone go all Lord Voldermort when it comes to naming the enemy?
I can barely surf the net... So now I am even more confused..
If you have a computer, or tablet, or smartphone, then it has a piece of hardware (these days usually a wireless component) that connects to the internet. That connection goes through a router or cell tower. But on a more granular scale, the device has logical "ports" that are like doorways dedicated to specific types of communication. Video uses one port, audio uses another, email uses another, web pages use another, bluetooth uses another, and so on, and so on.
One of those types of communication is called Remote Desktop Protocol. If it's enabled on your computer, and the right software is installed, then I can take control of your computer from wherever I am. I can look at your files, run programs on your computer, and even destroy your computer if I'm creative enough.
Other methods of connecting would allow me to remotely spy on what you're doing. Others would let me modify some files and what they do.
To prevent me from doing that, your computer has the ability to close the ports/doors that shouldn't be in use. You can also have a device on the network that you connect to (a Firewall) that can further restrict what can get in and out of the network.
In theory, Black Magic is an app on your device that if you choose to use, it opens ports. If the Hacker named Beside has gotten inside the Network, and finds the ports open, then he can try to install more software through them that will give him more and more access to your computer.
Or, if you're unlucky, someone can plug a device (usb stick, memory card, etc) that has the Hacker software on it, and that software is installed locally and gives over immediate access to all those ports that were previously closed for your safety.
In the magical sense, Beside does the same thing, but with the spirit/mind/body. Black magic opens a path into the mind, which he can see through or listen or whisper. The more he whispers, the more it opens to him, until he can fully possess and control.
Or, using a vector like the Athame, the paths are opened without the slow infection and without consent.