Mine is Black Hammer. I have never gotten it or seen it drop in any of my runs. Also it's the only Crota weapon I don't have.
Edit 1: For all of those people who are saying Thunderlord, just a few hours ago I got it in IB.
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⊂_ヽ \\ \ ( ͡~ ͜ʖ ͡°) < ⌒ヽ 2 / へ\ F / / \\ A レ ノ ヽ_つ B / / 4 / /| U ( (ヽ | |、\ | 丿 \ ⌒) | | ) / `ノ ) Lノ (_// ~Fab Warlock