ien
This is a relative pronoun that is only used with the relational se. It is used to expand upon the main object of se. For example:
iÄ“lte teteñ anþēŋŋeni ien la ma pa antÄken anÄnexa makēñ;
The main object of se (here realized as teteñ, which is se in the past tense with a 3rd person paucal reflexive inflection “they, to each other”) is anþēŋŋeni “argument”. ien modifies anþēŋŋeni to tell us what the argument was: who is the strongest?
iÄ“lte teteñ anþēŋŋeni ien la ma pa antÄken anÄnexa makēñ;
Long ago they, to each other gave argument: who is the strongest?
On to the next sentence in the North Wind and the Sun.