Chieftains

Chieftains is a technology unique to the Vikings that can be researched at the Castle in the Castle Age in Age of Empires II: The Forgotten.

This technology gives the Berserks an attack bonus versus cavalry of 4.

Strategy
Berserk as an infantry is a very durable unit, with chieftains it also become versatile as they can now counter cavalry units. Researching Blacksmith technologies before should not be disregarded.