To prevent the awakening of the Warring Triad, Bismarck must be cast down once more, and Unukalhai declares that you shall slay him using the same methods as before. The boy then furnishes you with a large gemstone and bids you show it to Chief Sonu of the Zundu, in case he is feeling reluctant to give you all that you require.
Chief Sonu believes the gemstone in your possession is a relic of great power. If smashed against a Vundu aetheryte nestled in the Whale's Crown, it will render Bismark vulnerable... Yet, upon further reflection, you recall no mention of this white whale's newfound invulnerability prior to this. That said, if the primal does have some form of protection, it will need to be circumvented before he can be defeated. Inform Cid of your new task.
Cid seems poised to explain in detail how Chief Sonu's request makes no sense whatsoever. However, he instead hazards that it might bring you good luck.
With all your might, you smash the gemstone against the aetheryte...and nothing much happens. Out of habit, you attune with the aetheryte..and again, nothing much happens.
If your actions had any meaning, it remains to be seen. In any case, Cid is waiting.