The Ulduar bow is from the Cache of Winter (Hodir).
The Cryptfiend Slayer is from Halls of Reflection (N). It's in the cache you get on the gunship after you escape, so RP-wise might have come from anywhere.
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Looking for it on Wowhead now...
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My first thought was, "That looks like Avalanche!" (recolor). Sure enough, it's a 'same model as'.
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Hmmm... Definitely like it more than the Artifact bow variant. Looks vey Titanish. But also kind of fat/bulky for my toon? What do you think?