Literary devices in the poem The Raven in stanza fifteen through eighteen?

There are different forms of figurative language, alliteration and onomatopoeia. Alliteration is reapeating the sound of a consonant over and over in a phrase. Ex. "And the silken sad uncertain rustling of each purple curtain..."

Other examples of alliteration

• Peter Piper picked some pickled peppers.
• Seven silver snakes slithered slowly.

Onomatopoeia are Words that sound like what they are. Ex. "Tis some visitor, I muttered, tapping at my chamber door..."

Other examples for onomatopoeia

• pop
• crack
• snap
• echo
• slap
• beating
• rustling

The bust of Pallas on which the raven alights adds to the ancient feel to the narrator's tale. His question, "Is there balm in Gilead?" alludes to Job from the Old Testament, who suffered the insufferable and hints at the internal pain suffered by the narrator on account of Lenore's death. This example of an allusion is an example of a figurative language.
For onomatopoeia: tapping, rapping, ah, rustling, beating, etc.
there are all sorts like alliteration or internal ryhme:

Alliteration- "grim, ungainly, ghastly, gaunt"

Internal Rhyme- "dreary/weray" "shutter/ flutter"

These are examples of figurative language in "The Raven"