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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Actually there are three places that have been glitched into way before the dark below was released plus over 6 ghosts found. There are two in kings watch, three in the vex spire, and two on the ketch ship hidden above the hellmouth. Plus some unconfirmed places like the drill on mars and a few minor caves and facilities hidden under the venus map. No ghosts found in the unconfirmed areas plus they have little to no development.