Hulls are black bards that were once good but have now joined the nameless one (sharma). They are bound to the nameless one but unlike most of his slaves they still have some will of their own. They don't die in the ordinary way but they can be killed. No-one knows what happens to them afterwards, but after living seven times the natural lifetime they become 'abhorrent to behold', as cadvan said.
But they have bodies like ours and can pass for mortals. No-one knows how many there are but it's thought that no bard has turned into a hull since the great silence.