ScourgeBringer

Hi !

For this achievement you need to explore all 100 rooms in a normal run (5 levels).

According to the devs,you'll have 2 secrets in a realm (level) if they have only 1 shop (instead of 2).For me it was level 2.
Be careful,you can have more than one realm with 2 secret rooms !

Also, I don't know if you can see some details for the secret walls. I just know that some light/dust pass through them in level 1.For the other levels I blasted every wall haha.
So simply blast every corner of every rooms where there's less than 4 exits (only your gun can destroy secret walls).So use wisely your ammo !
You can also try to buy a better gun,it's recommended for this.

If you have the map of the level, you're good for this one.

Good luck and have fun !

JocofAllTrades here with my first Achievement Guide!
(Minor Spoilers)

To Clarify the achievement, You must explore every room in a playthrough of Scourgebringer
(I unlocked mine finishing the Original 5 Realm campaign and accepting Judgement)
"Everyroom" includes Secret rooms, Stores, Challenge rooms (I don't think their completion is needed but it is worth completing them for resources) and everything in between.

Important notes- There will always be 3 Item rooms (Stores and Secret rooms), Stores will have a Blood drop symbol on them in your map whereas Secret rooms will appear as Checkerboarded rooms once discovered on your map. Don't waste fury on a realm boss But Sub-boss rooms can contain a secret entrance.
(Fury needs an enemy present to activate.)

You can* lock yourself out of this achievement in Realm 5 if you face the final boss before fully exploring its realm as his dialogue will force you to end the run, Saving and quitting will not resolve this ultimatum. (Important to note as you can use earlier bosses to regain ammo and fury upon them being unlocked) Rooms with 4 Exits cannot have a secret wall however, You can use these free rooms to recover ammo and fury. Bloodpoints are best spent on Ammo reload speed and Fury recharge speed. I actually opt'ed into a Chaos MAX rooms (25 Minimum rooms) to increase the resources i have in the form of many more 4 entrance/exit rooms, enemies for blood points and potentially a higher chance at 2 shops only leaving 1 secret room.

How to find Secret rooms- Unfortunately after Realm 1 it was impossible for me to figure out any obvious sign of the presence of a secret room. Shooting the entrance twice with the starting pistol is enough to reveal the entrance whereas activating a fury and attacking seems to always reveal their entrances. (In Realm 1 there is a fairly obvious dust/particle effect lingering on the entrance)

Achievement Solution- That said, During an active room your priority is to shoot first (At possible entrance locations) and using enemies as a resource to reload your gun. Always use fury immediately to reveal any entrances in addition to killing the least amount of enemies to refill more of my fury for its next use. Maps are a freebie for an entire realm, They reveal all secret rooms leaving the clearing for you to do. A quick check up for confirmation on your map would be 2 Blood drops + 1 Checkered board or 2 Checkered Boards and 1 Blood drop in regards to the Secret rooms. If you play it smart you'll always have the confidence to discover the secret rooms even if the entrance is in the boss entrance room. If you play loose and sloppy you can miss several rooms and essentially void the achievement due to not knowing where to look before your resources are depleted.

Hope this helps! If you need help with an Achievement i unlocked message me about it!

To unlock this you will need to go into every room on the map in every realm. Included in this are the secret rooms.

In each realm there is 1 secret room to be found. To identify a secret room there will be a slight blue sparkle on the wall where the hidden door is located. The sparkles are hard to see in realm 2 so keep your eyes open for this.

Shoot at the blue sparkle with your gun twice and the door will appear.

Beat realm 5 boss and finish the game and the achievement will unlock.

Note:
In the comments it's stated that there may be 2 secret rooms in a realm that only has one shop. I myself never saw this but something to keep in mind if you get a realm with only one shop.

Master Explorer | 20 GS | Est. Time 3 Hours
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To assist people, I decided to link some photos of the rooms I found. All rooms, in my experience, had light blue “sparkles” around the hidden room door. These sparkles will can be extremely hard to see/find in the later levels.

I would recommend using your “Fury” attach in rooms that you believe might have hidden doors. This will reveal a lot of hidden doors. You will need enemies in the room to use fury.

Also as stated in the other solutions and comments for each level the number of hidden rooms and stores will always equal 3. Therefore, you will have 2 shops/1 hidden room or 2 hidden rooms/1 shop.

--- Happy Hunting ---