Stanza 5 Summary

Get out the microscope, because we’re going through this poem line-by-line.

But evil things, in robes of sorrow,
Assailed the monarch's high estate.

Okay gang, maybe you saw this coming, but here's the big switch in this poem. Everything about the palace/head was going just great for a while. Now things turn bad.
At the beginning of this stanza "evil things" come out of nowhere, wearing "robes of sorrow." Those sad robes are a classic Poe image. They're a little hard to visualize, but we sort of feel what they must be like.
(We imagine these "evil things" looking a little like the Dementors in Harry Potter.)
The evil things attack ("assail") the glory and the harmony of the king's life ("the monarch's high estate").

(Ah, let us mourn!—for never morrow
Shall dawn upon him desolate!)

Here's another one of the speaker's little moments where he sets a thought apart with parentheses, and steps out of the flow of the poem a bit. Suddenly, he's inviting us to feel sad "mourn" along with him for the miserable, abandoned ("desolate") king.
We had been talking about the distant past, but suddenly we're in the here-and-now.
The speaker imagines that the king will never see another new day ("never morrow / Shall dawn upon him"). The idea that this poor king will never see another sunrise is meant to symbolize the unhappiness of the king's current state.
We've gone from happy and harmonious to sad and lousy real quickly here, haven't we? Bad times indeed.

And round about his home the glory
That blushed and bloomed,

For just a moment here, we're back in the old, happy world we started out in—yay. The "glory" of the palace flickers back into view, helped out by the vivid alliteration "blushed and bloomed."
That line makes the old palace sound like a fresh new rose. But by now of course we know the bloom can't last. In reality, it's gone forever.

Is but a dim-remembered story
Of the old time entombed.

After that brief little break, we come crashing down into the dark and sad world of the present day. The beauty of the palace is just an old story now, from a time in the past that is so lost and gone, it's like a dead body shut up in a grave "entombed."
We think one of the most important things to check out here is the shift in the imagery and the word choice in this stanza.
Before, everything was "sweet" and "gentle" and lively. Now things are "dim" and "old" and generally a huge bummer.