'Tis the vault of thy lost Ulalume!" (line 81)
The poem keeps us in suspense, holding off on letting us know where all this sadness is coming from. Finally, at line 81, Poe drops it on us. This is the big reveal, the moment when we learn that the speaker has been destroyed by grief for the lost Ulalume.
Then my heart it grew ashen and sober As the leaves that were crispèd and sere— (lines 82-83)
We started the poem with sad leaves under a sad sky, described with the words "ashen," "sober," "crisped," and "sere." Now that image comes back, but this time the sadness has spread to the speaker's own heart. We think this line really shows how horrifying it was for him to realize where he was, to remember the loss of Ulalume. If this poem started out sad, things have only gotten worse. That's trademark Poe. He doesn't want us to simply recognize the sadness, but to soak in it for a while, to almost enjoy it.