Add a monster spell that makes a nearby other monster berserk. The behavior for a berserking monster is unspecified, but presumably involves it attacking anyone (besides the caster?), or attacking the player, with no regard to whether it was previously peaceful or tame and no concern for its own health.

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.

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.

Add 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. This idea was 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 should get a berserking bonus if their pet gets killed.