The Sierra Riddle - This Is What I Just Did.

[BBP]

? The protagonist in Mystery House visits the graveyard?

[AndreaDraco]

No

[Tawmis]

King Mark from Troll Cave (Al Lowe?)

[AndreaDraco]

No. It's more famous than that

[Tawmis]

Patti in LSL3 (though I am not sure about the mould/mold)... nor the "down here."

[Rudy]

Connor in the Realm of the Dead in Mask of Eternity?

[AndreaDraco]

No and no.

The main character is in an underground place and there is actual mould where he is.

[Tawmis]

Roger Wilco in Space Quest 2?

[BBP]

Gabriel in the snake mound in Sins?

[Datadog]

King Graham in Mordack's Dungeon in KQ5?

[AndreaDraco]

No to all.

Another hint: the protagonist is in a sort of underground labyrinth, and he must use an object to correctly navigate the maze.

[Tawmis]

Roger Wilco in Space Quest 5?

[MusicallyInspired]

Nobody said Alexander in KQ6 in the catacombs yet? He uses the hole to look through walls...I can't remember if it's for finding your way or not, though.

[AndreaDraco]

No and no

Another hint: There are murals on the walls and the object the protagonist use fits in his hand

[Tawmis]

I will let someone try to guess this one, because I have extended my brain too much.