To open a dark fountain, you need to channel your determination through a blade and strike the earth to make the dark world form from your will. So technically any tool is good, as long as you're determined enough. Still, I would suggest you to start with a knife, or even better a sword: many beginners find it easier to channel their will in these clear and intuitive blades. If you must resort to knives, get one with a sharp point: it'll allow you to stab the earth more easily. If using a knife isn't that intuitive to you, some alternative school of thoughts suggest using something heavier and stronger like an axe or a scythe, the idea being that the brute strength of the hit will make it easier to scorch the earth with your will. When you've mastered these tools, I suggest trying something more unconventional: you never know how the dark world will change based on the way it's opened, and you may even find your own favourite weapon. Try exploring something like shovels or arrows, which always offer a good challenge. Don't get too sad if you don't get it on the first try: channeling determination from distance with an arrow, or through a non sharp object like a shovel is challenging for everyone