The eerie mood is created initially because of the subject matter of the poem. The character of Death is introduced in the first stanza, and since the 'd' is capitalized, it's clear that Dickinson is personifying death into an actual person. Thinking about Death in this way, as someone who stops to pick you up and having to get into his carriage to take the ride away from life is creepy in itself. This eeriness is supported when she uses the line "The dews grew quivering and chill." The words quivering and chill help to strengthen the eeriness that the subject matter creates. She also refers to the setting sun, which again helps give the poem an overall spooky feeling.
He is the character foil for Rainsford, since he got in the way of Rainsford's plans.
