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Banes
You aren’t the only manifestation of the void to have come into this universe. Each of these banes represents a threat to your supremacy. Take three lesser banes or one greater bane by default. Each additional lesser bane you take grants you two extras, a lesser ability of any kind, or two extra summoners. Upgrading a lesser bane to a greater bane allows you to upgrade a lesser ability to a greater ability.

Void-Touched Element (Greater): The void has touched an element that is easily accessible to the humans, one that occurs in significant quantities, and possibly one the humans already use in their weapons. It is also more dangerous to you, with mere skin contact burning you like acid, and it carries a significant risk that you could be destroyed if you are not cautious.

Half-Breed Betrayer (Greater*): The child is not just the offspring of any Elder God, it is yours. It knows your strengths and weaknesses, it is significantly more powerful than a lesser half-breed, and it has a personal vendetta against you. It will teach the humans all it knows, and it will lead them against you again and again until it perishes or you do.
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I've decided the void touched element should be titanium, since I figure that'd be used quite a bit in futuristic construction.