Leigh Hunt has used several figures of speech and other poetic devices in his famous poem “Abou Ben Adhem”. Here’s a list of those I have found out so far.

Assonance and Consonance:
Assonance is the repetition of vowel sounds in nearby words and consonance is its consonant counterpart.

Abou Ben Adhem (may his tribe increase!) — Assonance
…deep dream of peace, — Assonance
It came again with great wakening light, — Assonance
Abou spoke more low — Assonance
…deep dream of peace, — Consonance /Alliteration
…who love the lord. –Consonance / Alliteration

Simile:
Simile is a direct comparison between two different things using ‘as’ or ‘like’.

Making it rich, and like a lily in bloom,

The moonlit room is compared to a lily in bloom.

Symbolism:
The poet has used symbols to suggest something more than the literal meaning of the words.

It came again with a great wakening light,

Here, “wakening light” not only suggests wakening from sleep but also the wakening of mind with the wisdom that the angel carried — Loving one’s fellow men is more virtuous than loving the Almighty Himself. That is why Abou’s name topped the angel’s list.

The phrase “making it rich” in line 4 has a similar symbolic meaning.

Archaism:
The poet has used archaic words in “What writest thou?”, “Nay, not so” “I pray thee, then” etc. to bring an old-world feel to the poem.