The Reaper Explained - Mechanics of Reaper Spawn- Dos and Do Nots

Other guides already describe the method, which is:
open 20 chests on floor 1, then save, then go to floor 5 via any method (having used a goho-m on floor 5 before the save is ideal), then open 1 chest and hear the rattle of chains to confirm that was the 21st chest, then hope the next box had a monster in a box, which will change to the reaper.
If you hear the chains earlier it's because you already had a chest count going, and you'll have to redo the whole process I think, because I don't believe Reaper spawns on floor 1.
You can always reset the count at the start of the farm to make things simpler, or you can farm on floor 5 instead of 1 for some chance to get it (you'll probably lose it if the 21st chest you open is the final one on the floor, don't know how that works).


What won't reset the spawn:
-moving between floors (or dungeons) before the 21st chest:
this means any chests BEFORE your 21st can come from ANY floors, ANY dungeon
-killing shadows before the 21st chest:
this may seem like a weird condition but I'll explain below; otherwise it's safe to kill any Shadows before your 21st without fear of resetting your Chest counter*
-opening a Golden Chest before the 21st chest:
so by now you should've encountered shadows in some chests; those chests are where the Reaper will actually spawn. After opening your 21st chest, any (possibly all) chests on your current floor that has a Shadow in them will turn into a Reaper chest. This means if you want to spawn a Reaper, it WON'T be in the chest that will ABSOLUTELY give you an item (namely, Golden Chests)

What will reset the spawn:
-cancelling the opening of your Reaper chest once you DO encounter it, then get into a Shadow fight:
if you do so, the reaper won't actually despawn - you'll still hear it the chains. This means the Reaper is STILL on your floor. HOWEVER, the chest you gave up on opening will then reset into a normal item chest, and you'd then have to track down the next chest on your current floor that MIGHT be a Shadow chest (which takes a lot of luck - you generally won't know which one it is if there are multiple chests on the current floor). Thus, if you went ahead and opened the chest you gave up on, the Reaper won't be there and the chest counter will be reset to 0.
-exiting the floor the Reaper chain sounds are on:
when you've heard the chains, it means you've opened your 21st chest, and so leaving the floor THEN does reset the chest counter to 0
-opening a non-shadow chest as the 22nd chest:
as outlined before, Shadow chests will turn into Reaper chests after you open the 21st chest - this means the chances are high that you'll pick a Shadow chest IF you know there's ALWAYS at least ONE Shadow chest that'll spawn on a specific floor.

{{editor's note: Not sure there are any guaranteed shadow chests but Yukiko floor 5 often has at least one, making it an ideal location as there are only 2 chests and they're down the doors/hallways closest to the entrance from floor 4. OP claimed there's always 1 shadow box here, but I've had 2 boxes with 0 shadows plenty times.}}

-Switching party members during your chest-opening process:
yes, after I opened my 20th chest I tried to switch out party members 'cuz I wanted freaking Yosuke to get an item instead of Kanji. Turns out, I come back to the floor I was on to open the 21st chest, and I couldn't hear ANY chains, no matter how long I waited. This means that the counter DOES reset if you switch out party members AT ANY TIME during your hunt.

*Chest counter, for clarification, is the counter in the game that counts how many chests you've opened until you encounter the Reaper.



ported from a forum thread at gamefaqs.gamespot.com/boards/641695-persona-4-golden/69669970