Hitting a monster with a headbutt attack while wearing the War-helm of the Dreaming causes that monster to be put to sleep.

Unicorns’ headbutt attack is presumably hitting you with its horn, so it should apply the regular effects of a unicorn horn.

If you are in a form that has a headbutt attack and are wearing a helmet, the helmet’s enchantment should be added to your to-hit and damage calculations.

Barbarians who go into berserk mode should start attacking multiple targets in an arc as if wielding Cleaver. They should also randomly intersperse headbutts in with their attacks.