Diablo/Black Mushroom

In my point, this is the longest quest in game, however, isn't that long.

At caves level 9, grab the Fungal Tome (it will be somewhere on the floor, there isn't a exactly point) and take it to Adria, the witch. She will say something about a ancient magic and that, in order to make use of the tome, she will need some black mushrooms.

Backtrack to the level 9 and search for the patch somewhere there. After getting them, give them to Adria, that now will say she needs an elixir from Pepin. Go talk to him about it and he will say that he needs a demon's brain to make it.

At level 9, kill a unique demon that will be somewhere there, specifically on an island surrounded by a river of lava. Get its brain and return to Pepin. He will make an elixir for you. So now, go talk to Adria with the elixir. She will say that she doesn't need it anymore.

Simply drink the elixir to make all your base attributes raise by one.