Barbarians who go into berserk mode start attacking multiple targets in an arc as if wielding Cleaver.

Berserking monsters must be below a certain fraction of their maximum HP in order to act berserk, rather than just being constantly berserk. The fraction (or just a threshold) is deterministically computed based on the monster ID.

A Barbarian rage mechanic (possibly an effect on Cleaver replacing its current damage bonus) where a damage bonus is assessed based on how long ago you last killed a monster. Originally a buff to Vorpal Blade where it beheads anything it hits if the last behead was N turns ago, but this is unbalanced.

Cavemen get a berserking bonus if their pet gets killed.