No matter what threats or injuries beset you outside, here you will be safe and healthy. Well, what passes for 'healthy' for you now. And if you really hate someone, bring them here. Trick them into coming. They'll get infected, one way or another; a lungfull of spore, a thornprick, a bit of residue on their hand. They will blood-vomit and the blood will have tiny centipedes in it. They'll shit out their own spinal fluids. Their eyes will milk over and hatch; little spines and brambles will grow from the sockets. They'll survive for months or years, doctors will be baffled, it will be completely fucking great.
That's all for starters. You'll learn more as you go. Much more. But if I told you everything now you might not do it.
Whatever you do, just guard it with your life, with your very soul. If you think you're in danger of loosing it, dig it up, kill it with a silver needle, let someone else make a new one some day. You'll feel as if you've pierced your own heart, but it's better than letting it fall into the wrong hands.
Because you're a Holder now.
And you'd better not let them come together.