But evil things, in robes of sorrow, (33)
Sadness appears suddenly in this poem, and in a really intense way. When the "evil things" show up, they are literally wearing "sorrow," as if their clothes were made of sadness. We don't know anything about what they are or where they came from. In fact, their sudden appearance, out of nowhere, helps to emphasize how random and unexpected the loss of mental balance can be.
(Ah, let us mourn!—for never morrow (35)
In this line the speaker pretty much comes out and tells us that it's time to be sad. It's an invitation to "mourn" but also kind of an order. It's like when a preacher says "let us pray." He's not making a suggestion; he's telling you what's going to happen next. So the speaker of the poem is letting us know that we're shifting gears here, and now is the time for mourning, almost as if the palace's king was dead.
Shall dawn upon him desolate!) (36)
The king, like his palace, is ruined, abandoned and completely filled with sadness ("desolate"). He's in such bad shape, and the atmosphere is so sad, that it's almost like he will never see another sunrise. This absence of any possible future for the monarch is part of the sadness of this section of the poem.