Here's the page from the translation of the tablet Kevin was working on. Sam reads part of it at the end of the episode.
Here's what I can make out:
"Waste not thy time nor your breath upon the Leviathan herd. Point thy blade at the heart of their master, for from him springs all their messages. Leviathan cannot be slain but by a bone of a righteous mortal, (as) light and good as the Leviathan are angry and dark. Washed in the three bloods (of) the fallen: a fallen angel, --- of fallen humanity, and --- of fallen beasts."
They already have the blood of the fallen angel from Cas.
So who are the others? Based on the promo, they find the Alpha vampire for the blood of the fallen beasts.
Fallen humanity? Ok, I was thinking Sam for some bizarre reason, but as casey28 so wisely pointed out, fallen humanity is probably referring to demons.
As Ruby told Dean in S3's "Malleus Maleficarum": "It might take centuries, but sooner or later hell will burn away your humanity. Every hell bound soul, every one turns into something else. Turns you into us, so yeah, yeah you can count on it."
I'm thinking the "righteous mortal" might be Dean. After all he was the righteous man who shed blood in hell to start the breaking of the 66 seals.