"The Raven" by Edgar Allan Poe is one of the most famous poems in American literature, and arguably the star of the poem is a symbolic black raven who appears to a grieving man. We know that the raven finally enters the room in stanza seven, and then he sits, perched on a bust above the door.
In other places in the poem, the raven is called an "ungainly fowl," an "ebony bird," and "a stately raven." These are short descriptions which could apply to any raven.
In the two stanzas you referenced nothing changes; man and bird are still sitting. However, in these verses we get a more detailed description of the visiting bird. The narrator has moved his chair so he is sitting directly in front of the raven, and then he really looks at it.
In stanza twelve, the narrator uses alliteration to describe the raven:
this grim, ungainly, ghastly, gaunt and ominous bird of yore.
In stanza thirteen, the speaker of the poem is still sitting and looking at the strange bird. Again he uses alliteration and describes it as
the fowl whose fiery eyes now burned into my bosom's core.
These two descriptions give us a clearer picture of the strange, symbolic bird, and he is depicted as evil. Note that nearly all of the descriptors are indicative of something rather demonic. This image is borne out in the following stanza, when the grieving man calls the raven a "fiend" and a "devil."
"What this grim, ungainly, ghastly, gaunt, and ominous bird of yore"
"To the fowl whose fiery eyes now burned into my bosom's core;"
If you need more help on this poem, do not hesitate to search The Raven by Edgar Allan Poe on eNotes.