Ebrietas, Daughter of the Cosmos is Every Bit as Obscure and Difficult as Bloodborne Needs for Great Ones

gamerant.com:

Hidden below the Grand Cathedral of Bloodborne sits Ebrietas, Daughter of the Cosmos, one of the game's most interesting and most difficult secret bosses. Many players will likely never even encounter Ebrietas. What makes Ebrietas most interesting is its placement, not being hidden for a mandatory reveal at the end of the game like the Moon Presence, but instead buried deep underground with the only access point being a hidden elevator.

Everything from the path to Ebrietas to the strange arena where the fight takes place makes for an impressive presentation that makes good on the godly claims of the Great Ones. The long trek towards the bottom of the Grand Cathedral is a perfect payoff for the way the lore builds up these cosmic deities, showing the rotting underbelly of the entire world of Bloodborne buried under the surface.

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In order to find Ebrietas, players first need to enter the optional area above the Cathedral Ward that leads through both an Orphanage and eventually back into the Grand Cathedral, right above the location of the Vicar Amelia fight. The area isn't available until late into Bloodborne's story, likely being one of the last places players will explore before heading to the Nightmare Frontier and reaching the end of the game. Strangely, though, getting to Upper Cathedral Ward and through to the top of the Grand Cathedral requires players to find a key in the Hyperion Gaol and backtracking all the way back to an elevator in a side room in Oedon Chapel.

So, not only is the area itself a secret, it is hidden in a side area that players would have already passed by hours beforehand and likely forgotten in light of the night progressing through to the reveal