To unlock this achievement, you will need to acquire the basic broken flail (moonraven trial).
When you acquire this, you will need to go up to the overview of the cave, (continue walking straight ahead when you leave spawn, instead of going right or left) From up here, you will need to throw the broken flail down towards the blood pool with RB, then pressing RB again to teleport to the blood pool.
Easy as that, now it is a bit harder to actually unlock the broken flail, especially because the guides were not really spot on imo. Besides, I have not found a video yet of all the spawn locations of the 3 gears and the 2 books. I know pretty much all the locations by now, though I do not have decent recording equipment.
1. First thing you will need to do is finding the 3 gears and 2 books on the floor, scattered around the map. I will try to describe all the locations I know of as best as I can.
When leaving spawn, head right. Just to the left of the access teleporter for the bossfight you can find a GEAR
. Continue into the morgue. You can drop down here. Just before you drop down, you can find a BOOK
on your left. In the phylactery, you can find a GEAR
at the edge of the map, immediately to the right of you when you walk down the stairs next to the large bloodpool. In the area with the blood altar, You can find a GEAR
to your right as you leave the teleporter, against the edge of the map. Now head over to the large blood pool where you bring the battery to (other exit from the phylactery). When you reach the plateau, a GEAR
can spawn immediately on your right, next to the stairs. A BOOK
book can spawn on your far left, hidden between some stones. Now head over to the area with the Planet-system and turn left up on the stairs. You can then go left again, a BOOK
can spawn immediately on your right hand side against the wall. Turn back and head dowstairs towards the gear room. You are now standing right in front of the pillar with books on it. Place the 2 remaining books on it to reveal a hidden room on your left. You can find a GEAR
in here immediately on your right. There is a last chance to find a GEAR
in a little alcove in the corridor downstairs, the one you can take from more or less here, coming across a teleporter and the gear room. In the gear room, you can place all 3 gears.
2. Next, you will need to kill zombies next to the planet-system until the planets turn purple, green, yellow and red. (15 zombies or so)
3. Now comes the hardest bit if you are doing this for the first time. Side note: MrRoffilewaffles as well as some other youtube guides I watched are totally wrong on how to do this step or simply make it much too complicated.
In the hidden room we found earlier, there is a projection of the 4 systems situated against the wall. Only downside, you will need to mirror projection basically. If the top is North, right hand side East, bottom South left West, mirror the projection so that North and South stay the same, but East and West swap basically. This means planets directly South and and North of the earth will stay in the same position, but anything else will be mirrored.
I've made a small example in Paint with before and after mirroring it:
Next, you will need to stop the 4 planets on the large system at the right time, so that they match the mirrored projection on the wall. The highest part which holds the sytem (more or less in the direction of quick reload, is North. You will need to stop all 4 planets correctly in one you. If you stop one at the right spot, you will hear a loud sound (almost like a large door being locked). If you fail one, simply stop the remaining planets by pressing X on them and try again. Keep in mind that it might take a second or so before the planet actually stops, after you've pressed X.
After completing this step, you have the basic broken flail, not enough for the moonraven trial, but enough to go for the go long achievement. (as described at the start of this guide)