In this part of the guide Minecraft Dungeons, find out how to reach the secret level???. This is a level with cows, a tribute to Diablo II's "Secret Cow Level".
To unlock this secret mission, you will first have to collect several runes. But to start collecting these runes, you must first complete the game a first time. Then return to your camp, enter the cathedral and interact with the slab at the back (picture1). For details about the cathedral, see the Camp walkthrough section.

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Once done, you must now replay all the levels of the game in order to find a secret passage leading to the runes. Levels are randomly generated, but rune-related items are still present.
Rune of the Creepers Woods level
When you need to free the villagers, go to the left to locate a large slab (picture2). Press the button in the middle of the nearby stones to open a passage (pictures3and4).

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The rune is at the end of this passage (picture5).

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Mire Swamp Level Rune
Just before escaping from the swamp, interact with the button at the foot of the column (pictures6,7and8).

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Enter the passage to take the rune (picture9).

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Cactus Canyon level rune
A button is hidden next to a palm tree in front of the room containing the blue key (pictures10and11).

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A secret passage opens behind the blue key (pictures12and13).

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Rune of the Pumpkin Fields level
When you reach the ramparts, use the button hidden behind the crates (pictures14and15).

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Take the passage that opens in the tower to find the rune (pictures16and17).

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Rune du level Mines of Redstone
When you need to free the second group of villagers, interact with the button (pictures18and19).

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Enter the passage that opens nearby (pictures20and21).

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Desert Temple level rune
When you need to retrieve the golden key, interact with the lever located behind a palm tree (pictures22and23).

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Take the passage that opens next door (pictures24and25).

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Burning Forge Tier Rune
As soon as you enter the cave, go immediately to the left to find a lever against the wall (pictures26and27).

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Enter the passage and take the rune (pictures28and29).

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Highblock Palace Level Rune
Before entering the castle to overturn the tables and ruin the party, interact with the lever on the top wall (pictures30and31).

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Take the passage and pick up the rune (pictures32and33).

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Obsidian Summit level rune
Enter the building and locate a library (picture34).

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Interact with the book to open a passage to the rune (pictures35,36and37).

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As soon as you have all the runes, return to the cathedral of your camp and interact with the back slab to open a secret passage (pictures38and39). Enter the passage and take the map (picture40).

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Then open the world map and select the level ??? (picture41). Progress by eliminating the cows that attack you (picture42).

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Before you can leave the level, you must fight a boss: Monstrous Mooshroom (picture43).

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To defeat him, you can for example stay at a distance and use your bow/crossbow (picture44). If you prefer to attack him in close combat, stick to him, hit him 4-5 times (picture45), then quickly go around the boss to reach his back (picture46), and thus avoid receive damage.

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Monstrous Mooshroom boss fight video: