There are two ways through the door: finding the combination and breaking in through the alternate present timeline, which awards the Temporal Investigator special action. The key objective is to enter Stilton's study and observe the events of the past that took place there. Optionally, you can open another timeline - an alternate present - which permanently replaces the original game state. Additionally, certain actions in the past will shape the present.

The key mechanic in this mission is the ability to move between the present and the past, allowing you to, for example, pass through doors that are blocked in one timeline but not the other. Supernatural powers are disabled here, although the Heart still functions. The map above shows the layout of Stilton's mansion here's the key-free clean version for your reference.

Painting: The Torturer’s Quaternionic Groan.Painting: The Commutative Rats and the Weeper.Note that the first time I did this the water didn't lower due to an odd bug, so make sure you save it back a few steps (perhaps before jumping into the water) in case you need to reload and try again.Dishonored 2 still has some tricks to play. The achievement should unlock about now (or when you pick up the rune). Turn the wheel and the water should subside, lowering a floating box with the rune on it. Swim down to the crank wheel, grab it, then head to the left to find the place where you can put it. Switch to the present and limb through the hole and into the water. Climb onto the box and look through the hole and throw the wheel into the small room there. Take out both guards, then grab the crank handle/wheel to the left of where the guards were. Once down the stairs, head to the right and in the past timeline you'll see two guards looking up at a hole in the wall and having a discussion about something. This achievement is earned in mission 7 'A Crack in the Slab.' After getting the time turner device (one of a number of Harry Potter-like things that show up in this chapter), you need to head to the basement via stairs in the left side of the main inner ballroom in the back-center of the house (the large multi-story room you would come to by going through the front door, straight ahead through the lobby and through the doors then down the short corridor).

Dishonored 2 mission 7 rune water