Where to find it?

On Grim Haven, we need to find 7 blue buttons and shoot them to activate the Secret level.

First button

On a side of a boat in the first area of the map.

Second button

After you disable the blue barrier, go to the lighthouse and there will be this lion face which will open the iron bars behind you.
On the wall of the lighthouse is the second button.

Third button

Continue through the level until you come to this broken wall and behind it is the third button.

Fourth button

After that, you will go to the sewer part of the level and there over the edge is the fourth button.

Fifth button

When you get out of the sewers go left and in the ruins there will be the fifth button.

Sixth and Seventh button

After the castle door puzzle, you will need to find a button that opens a secret door and leave it for now.
Continue through the level until you come to this room and on the right side will be glass without the iron bars shoot it, and behind is the sixth button.
Again continue through the level and you will find this globe thing. And it will say that invisible paths were revealed.
Now go back and through the secret door we opened before and through a portal.
There will be a path made from white star particles and at the end there will be a secret area and on the side of a broken wall will be the last button.

Secret Level

Eventually, you will get on a boat where you will find a path from white star particles again which leads to the Secret level.
Go down the path and the End level gate will emerge from the water.