Here once, through an alley Titanic
Of cypress, I roamed with my Soul—
Of cypress, with Psyche, my Soul.
These were days when my heart was volcanic
As the scoriac rivers that roll—
As the lavas that restlessly roll
Their sulphurous currents down Yaanek
In the ultimate climes of the pole—
That groan as they roll down Mount Yaanek
In the realms of the Boreal Pole.
Our talk had been serious and sober
But our thoughts they were palsied and sere—
Our memories were treacherous and sere;
For we knew not the month was October,
And we marked not the night of the year—
(Ah, night of all nights in the year!)
The use of “treacherous and sere” in this line, while creating a repetition with the previous line (both ending with “sere), also serve to establish a sense of unfamiliarity felt by the speaker.  The line goes deeper that mere unfamiliarity by stating that it is "Our memories” that “were treacherous and sere,” which adds a layer of potential recognition.  The speaker may have been there before, which creates a foreshadowing of revelations to come.  Because the tone of Poe’s language is foreboding, hence “treacherous” rather than “unclear” or “foggy,” it can be expected that the foreshadowed revelation will be a negative one.
Ulalume