Okay, I need to actually try and explain because my mind is all over this right now:
Okay, so, context: I play an artificer who has a claim to fame of developing magical batteries for storing energy, spells, and what have you. We're currently in the middle of a world-threatening event where we have to gather these stones to defeat an invading threat from the feywild. We're in the middle of fighting the guardian of the spring stone, which is a large construct of roots, stone, fungi, and insects.
Our satyr ranger (the legendary Sheng), has noticed that the guardian doesn't attack unless it is under threat and convinces the party to try and reason with it instead. Through great effort, we manage to find out that it was tasked with protecting this stone, without which, it will die. However, it understands that we need the stone and is willing to give up its life to help us, as it views death as a form of change. We do learn, though, that it fears dying.
We're standing there, contemplating how to do this, since we need the stone to save the world. It seems like we're going to have to let this creature-- an entire ecosystem unto itself-- die to save the world.
Except... I'm an artificer. I understand magic and, most importantly, batteries. I put forward the idea that I could try to create a replacement core to keep the guardian alive and get the stone at the same time.
Our DM says that, in order to do this, I will need to give up one of my highest level spell slots PERMANENTLY and pass a very hard tinker's tools check. They describe each level of success; DC 20: We create a core that will eventually run out of power, and the guardian will die, only extending its life. DC 25: We create a core that can recharge itself, but won't have enough power to keep the guardian alive at the moment, and it will go into hibernation until it's ready. Finally, DC 30: We create a core that can recharge itself and completely sustain the guardian as though it were part of it.
So, we get to dividing the work among the party. Half of us work to convince the guardian to give us the stone, while the more magically inclined party members work to build the core. I'm given bardic inspiration (+d6) and help (advantage) on top of my own flash of genius ability (+4). I have proficiency with tinker's tools, and a high intelligence modifier, so I get double proficiency bonus (+6) and my INT (+4). In total, I have to roll (2d20(advantage) +14 +d6) and, hopefully, get over 30.
I roll a 7 and a 17, then a 3 on the d6. Which gives me a final roll of 34. The highest I've ever rolled on an ability check in D&D. We successfully cobble together a new heart for this creature out of a battery, some components, and plucking literal sunflowers from the field for the fey equivalent of solar energy. And it works. We retrieve the spring stone, and spare an innocent creature its life.