In Dream's castle, at the heart of the Dreaming, hangs a portrait. At first glance, it appears to be of the current Dream King, the "young lord dressed in white" who was once Daniel Hall. But on closer inspection, there are certain differences. Most notably, this figure is dressed all in black.
The new ruler of the Dreaming commissioned it, from one of the many artists who visit his realm nightly. It depicts his predecessor, the one known as Morpheus, who, in allowing himself to die, saved the Dreaming from possible destruction.
Dream feels such sacrifice ought to be honoured.