Being late to the game, the fourth ghost located in the tower has become impossible to obtain. Now that iron banner is held in the iron tower, saladin no longer opens the doors in the tower for guardians to collect the final ghost. If there is a way to glitch back there please leave directions. Unless Bungie, you are able to relocate this ghost for those of us who are collectors.
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There are several grimoire cards that newer players won't be able to get, this is just one of them. It may open up in the future; it may not; the only way to know is to wait and see.