Shikabane are the monsters from the Shikabane Hime franchise. They are similar to the Hollows you'd find in Bleach. They can only be killed by destroying the head.
When a person dies with regrets they will sometimes turn into a Shikabane. The Shikabane will then go on to cause as much destruction and devastation as possible. It's the job of the Shikabane Hime to kill these monsters.
Shikabane can also be born from a person's "nature" after they die. Regrets are something people have right before they die but Nature is something the person lives with throughout their entire lives. A Shikabane born this way seems to much stronger and smarter than the average Shikabane.
There is also a very rare case where a Shikabane named
Hokuto was born without Nature or Regrets. This seems to have made her the strongest Shikabane.