Crisis Core Final Fantasy VII: Reunion has plenty of missable Trophies/Achievements, especially throughout the main story. The Cage Opener is the final such Trophy/Achievement in Chapter 10.
Opened the path to the Cage of Binding.
Chapter 10 is the final Chapter of Crisis Core Final Fantasy VII: Reunion and as such, features the game’s final dungeon. One you’re inside, you’ll eventually reach the Depths of Judgement where you can unlock the way to the Cage of Binding and earn yourself the final missable story Trophy/Achievement in the game.
How to Open the path to the Cage of Binding in the Depths of Judgement
You must first progress through the area, and you’ll eventually reach a Save Point in the Depths of Judgement, this will serve as your go-to point to for the first part of this sequence. You’ll find three additional exits out of this area (on top of the one you used to reach here), with the entrance to the Cage of Binding in the Lake of Oblivion to the south.
Don’t rush off to there yet, however, as there’s a small task you must take care of first. If you look to the right of the Save Point, you’ll see a tablet which, when examined, will have text from LOVELESS - Prologue. Your goal is to examine this, and three other tablets scattered throughout the area before you head to the Lake of Oblivion. Below, you’ll find a list of where you can find them all:
LOVELESS - Prologue
This is a simple one, you should find yourself right next to this tablet if you’re at the Save Point in the Depths of Judgement. Examine it to read the text and then move on to the others.
LOVELESS - Act I
You should arguably make this tablet your last one, since it sits outside where you need to go after. Regardless, you’ll find this tablet in the cave that leads to the Lake of Oblivion to the south.
LOVELESS - Act II
Leave the Depths of Judgement to the east to reach Howling Fang and you’ll find the tablet for Act II at the end of the path in front of you, by the locked gate leading to the Behemoth King. Examine it and move on to the next tablet.
LOVELESS - Act III
The final tablet can be found by leaving the Depths of Judgement to the west to reach the Portal of Severance area. Inside, you’ll find the tablet for Act III right in front of you.
Lake of Oblivion
Once you’ve examined all four of the tablets, enter the Lake of Oblivion found to the south of the Depths of Judgement. If you look closely here, you’ll see five areas here that has a crystal above and a slight glow on the water below it. You can interact with these and a will drop down. Defeat it, and the enemy will die on that glow from before.
Repeat this for all of the five glows in this area and you’ll unlock a new path to the Cage of Binding, an optional area of the final dungeon with plenty of chests to pillage. The Cage Opener Trophy/Achievement will also unlock at this point, which is the final missable story Trophy/Achievement in the game. If you’ve made it this far and got them all, give yourself a pat on the back!