A 'hagsfiend' is the fictional creature in the series Guardians of Ga'Hoole. They are part crow and part owl. They have eyes in which yellow light flows when in battle, called 'fyngrot'. The fyngrot can paralyze an owl and make it plummet to the ground. They are twice the size of the biggest owl, such as, a Great Horned.
A hagsfiend's feathers are dark and ragged, and they, unlike owls, do not fly silently. Their feathers make a hissing noise, 'chopping' the air. Their feathers carry no oil and when in contact with sea water, it drags them right in and the sea swallows them up.
In their feathers, half-hags lurk; they eat the mites and little creatures that rummage around in a hagsfiend's feathers. In their beaks carry poison, but hagsfiends are immune to it.
Hope this helped! :)
~Ruby