Bring the Bank Voucher to Idriana in the bank of Darnassus.
[player], your help has been immeasurable. The time will come, no doubt, when I'll ask for your assistance against the silithid once more. We must absorb what we have learned in Un'Goro; I fear that my assumptions that the silithid originated from Un'Goro are now wrong... that indeed there is a far stronger presence somewhere.
Take this note to Idriana at the bank. She'll give you something from my vault that should be viewed as a token of my gratitude. Thank you for your assistance, brave [class].