When Ava Cadavra arrives in the Penthouse, the first thing she asks is that you excavate the Bovine Shrine. It costs 2500 simoleons to build. Once complete, it has little to no use in the game, as it is only used after Ava has left the Penthouse in order to keep Horus Menhoset IX alive by giving Gourds to the Egyptian Statue. Despite the Bovine Shrine being compulsary to build in order to progress in the game, it still rewards you with a bonus hotel score of 6% when built.
Unlike most other rooms in the hotel, the Bovine Shrine's entrance doesn't become visible until construction has started. Once your sim has purchased the permit from City Hall and it has started being built, the entrance to the shrine will appear in the large boulder on the right of the main basement stairs. Prior to the construction of the Bovine Shrine, it's simply a non-interactable part of the basement.