There are no Key tiles in the Advanced Game. Instead, you will need to acquire Keys from within a Keeper. Keepers have Keys inside of their ethereal bodies and are the only type of Monster whose presence persists even if they are not Illuminated. They are permanent additions to the board until their Key is removed.
When placing a Keeper, the Mouth of the creature always faces you. They attack much the same as Wax Eaters, but are not triggered from behind. Similarly, they only strike in three directions and cannot strike behind themselves. Being hit by a Keeper at range (as opposed to landing on it) forces you to discard 1 tile from the Draw Stack.
The ranged attack of a Keeper cannot be Blocked using Nerve.
If a Keeper is hit by another monster from behind, it does not cause a chain reaction.
Moving onto the Keeper earns you a Key, just like a Key tile would. Approaching from the rear, where the monster cannot attack, is the only safe way to get a Key and incurs no penalties. However, if you spend 1 Nerve to CHARGE, you may move onto a Keeper tile from a direction other than the rear. Doing so is not possible without spending Nerve, but it does come with penalties.
Moving (or Falling) onto a Keeper from any direction other than the unprotected rear causes you to go Lights Out and discard 3 tiles from the Draw Stack. This 3 tile attack CAN be Blocked by spending Nerve. A Falling prisoner without a Key can take the Key as they land. Once the Key is collected, the Keeper immediately becomes a Pit. The falling Prisoner must scramble onto an existing adjacent tile or a newly drawn one placed adjacent to the Pit.