The THOR DOOR is directly below the ATLAS DOOR. To get there, climb the first two low steps onto the central structure. Turn left, hop down, then turn around and safety drop to the level below.
There are now 2 lions (22-23) down below. You can probably shoot them from here, but it's also fine to wait.
Take a carefully angled running jump over to the walkway leading to the THOR DOOR, or go around to the left (there's a save crystal here in NG+) and approach from the other direction.
Proceed slowly into the room until the disco ball on the ceiling begins to shoot bolts of lightning. It's possible to be struck by lightning and survive, but if you can get through this area without taking any damage, you'll unlock the "You Are Really Loki" achievement/trophy. To do this, stop on the lowest step. Don't step down onto the flat floor between the first two gray tiles. If you do, Lara will get zapped.
Instead, turn right and step down near the wall. Go forward, keeping the wall fairly close on Lara's right. Turn left at the corner, so the lightning ball is ahead on the left. Walk slowly toward the gray floor tile ahead.
Stop just before the seam in the floor so there's one regular floor tile between Lara and the gray tile ahead.
Now take a diving roll over the gray tile by holding Walk and pressing Jump + Forward. Lara leaps into the air...
...swan dives to the floor, somersaults...
...and comes up standing. Release Walk and quickly run or jump forward into the next room.
NOTE: At this point, you're halfway to the Loki achievement/trophy. You must also pass through the lightning trap again on the way out.
Now comes phase two. Above is an enormous hammer. On the floor is a pressure pad.
To unlock the "Thor's Mercy" achievement/trophy, position Lara slightly to the right of the pressure pad and facing it. (You're ultimately aiming for the far wall just right of center.)
Run onto the pad. Count two (or listen for two squeaks followed by a crack) then run off the pad before the hammer falls.
Run forward but stop on the tile that's just to the right of the room's center line and just before the tile that touches the wall. When the hammer lands, 2 stone blocks fall from the ceiling. One should land here, on top of Lara.
It magically doesn't harm her, and the achievement/trophy should pop. (The other block lands on the ledge along the right wall.)
DARK MODE: Even with brightness adjusted, dark images on a light background can be hard to see. May I suggest a reputable dark mode browser extension, such as Dark Reader? (Download only from trusted sites, like darkreader.org or your browser's web store.)
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