Onslaught buff grants 20% increased Attack, Cast and Movement Speed. It has 4 seconds duration.
Ravenous Horde is a notable passive skill that grants increased damage, increased cast speed, increased attack speed, increased movement speed for minions, as well as a chance for minions to gain the onslaught buff on kill.
Ravenous Horde location: in the top right of the passive skill tree.
Anoint oil for Ravenous Horde: Opalescent Oil + Opalescent Oil + Verdant Oil