If you patrol on the Cosmodrome you can glitch into a supposed mission of House of Wolves where you can kill an Ultra Captain (if you don't know what I'm talking about look it up). Somewhere in the area you can find a dead ghost and get the Grimoire for it. I'm wondering how many other curious guardians have found it. My friend told me about it and it sounds cool.
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I've done it and it looks pretty cool