Known first to the group as the infamous "Book Guys," Atlas is a strange, metaphysical being who seems to hold a great deal of power in this world. He can take on a great variety of physical forms. Though Atlas seems to lack both understanding of and connection to the living world, he does seem to feel strongly for Willow.
Powerful as he may be, Atlas appears to be at odds with the corruption. His inability to defeat it on his own seems to be why he has brought other beings, such as our group, to this world.
The Horned King is known to be among the most powerful bringers of corruption. He is sinister and clever, reaching out to vulnerable souls in hopes that they can help to release him into this realm. Doing so, however, would be sure to devastate all who inhabit it.
John and Snow inhabited this world some time ago, and died after failing to cleanse the realm of corruption.