The Berserker's Brew is one of Plague Knight's potion Arcana. It is found in Stranded Ship and is traded for the War Horn or bought for 9000 gold. Once Plague Knight has taken a drink of Berserker's Brew he no longer takes knockback from being struck and deals damage to enemies by colliding with them whilst the potion is in effect. If Plague Knight is attacked whilst drinking he drops the potion and does not get the buff.
It is worth noting Plague Knight still takes damage from colliding with enemies, just no knockback.