Whitewalker

Whitewalker can't be destroyed excep't by creatures with Valyrian Steel.
They're never far, you know. They won't come
out by day, not when that old sun's shining, but
don't think that means they went away. Shad-
ows never go away. Might be you don't see them
but they're always clinging to your heels.
-Tormund Giantsbane