Final answer:
The sentence uses a metaphor, comparing pride to a seed that grows into two vines representing its dual nature.
Step-by-step explanation:
The literary device used in the sentence "I did not know then that pride is a wonderful, terrible thing, a seed that bears two vines, life and death." is a metaphor. A metaphor creates an implicit comparison without using the words 'like' or 'as'. In this case, pride is likened to a seed that grows into two vines, representing the dual nature of pride with both positive ('life') and negative ('death') consequences. This comparison does not only depict pride as a seed but also carries symbolic meaning about the inherent duality in the characteristic of pride. The seed metaphor suggests a beginning or origin from which something grows, thus attributing to pride a buried potential that can develop in multiple directions.