Hagsmire is the owl hell, where the souls of evil owls go to when they die. St. Aegolious Academy for Orphaned Owls is considered the closest to hagsmire that you can get without dying, due to its horrible conditions and unspeakable cruelty.
It is mentioned many times as the place where hagsfiends come from and currently rule. In the The Outcast, Coryn dresses up as a hagsfiend and threatens to send the Pure One guards to hagsmire and torture them so that he can save the egg. In The Coming of Hoole, Ullryck is said to come from the deepest smee hole, which is thought to be connected to hagsmire. Coryn also remarks that he would rather have his parents vanish into nothingness than go to hagsmire.
Known Owls Who Are Considered to Reside in Hagsmire