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Click here for a high-level overview of Outsiders
Little is known about the Outsiders (and any attempts to learn more tend to be met with extreme prejudice by White Council wardens).
It takes wizards centuries to develop defenses against Outsiders and magic apparently has little effect on them. They can be defeated by, among others, the powers of the Knights of the Cross. It is believed that Outsiders cannot actually be killed, only damaged sufficiently as to require long years to recover. Wizards are terrified of the Outsiders and of their power; however, as only mortal magic can summon an Outsider (neither vampires nor faeries can do it), the temptation to gain power can be irresistible. It is rumored that Outsiders can grant great magical power and can even offer protection from all but the strongest mortal magic.
Outsiders pose a very real threat not only to any who interact with them, but to reality iteself. Left free to act in our world, a single outsider could eventually destroy the world. The Seventh Law against Outsider summoning is unique among the Laws in that one Senior Council member of the White Council is assigned full-time to enforce it.