Esse dano só pode ser aplicado se for além do dano normal. A palavra chave é, como você notou, a parte extra . Veja como os danos extras são definidos:
Extra damage
Many powers and other effects grant the ability to deal extra damage. Extra damage is always in addition to other damage and is of the same type or types as that damage, unless otherwise noted. Because of this rule, an effect that deals no damage cannot deal extra damage. However, a power doesn’t necessarily have to hit a target to deal extra damage—it needs only to deal damage to the target.
Example: Valenae the cleric might have an ability that causes her to deal 5 extra radiant damage to undead creatures. That ability means she deals 5 extra radiant damage whenever she deals damage to an undead creature.
Ênfase adicionada. Você só pode usar Furious Assault em combinação com um ataque que causa dano sozinho.
Esta entrada está listada no D & D Compendium e, em alguns livros: o PHB3 na p220, o Compêndio de Regras na p223, MM2 na p217, e o Monster Vault: Threats to the Nentir Vale na página 123.