Work Text:
The Nightmare Heart (and, by consequence, Nightmare King Grimm) from Hollow Knight is a valid warlock patron for D&D 5e, as are many higher beings. (Pale King? Celestial! White Lady? Archfey or Celestial! Radiance? Celestial or Hexblade! Shade Lord? Fathomless or Great Old One!) Why is this exactly? Because the Fiend subclass exists.
Yes, the Fiend subclass is meant for the Archdevils or the Demon Lords, or powerful succubi and incubi, but if you think about it, the Nightmare Heart is similar in nature to an Archdevil or Demon Lord. It even grants powers in-game, as seen by the Grimmchild charm.
In fact, you could use the Grimmchild as your reason for adventuring. You could have it so that if you don’t collect enough flames to upgrade the Grimmchild by a certain time, you Get Die. And those flames could be spread out all over the map, at the DM’s discretion. Or you could use the Grimmchild as a fragment of power from the Nightmare Heart, and as its power increases, yours does too, leading to unlocking things like more invocations and more than one spell slot.
Also at the DM’s discretion, you have the option to opt out of the pact at any time, but this requires finding the Nightmare Lantern, and with a member of the Grimm Troupe willing to help you banish.
A mask, like those of Brumm and Divine, could also be used in your pact. A mask made Brumm stronger, and likely gave him some powers not unlike those of Grimm, though much weaker.
A musical instrument is the natural direction of the spellcasting focus, perhaps having a precious stone embedded in it, to act as the spellcasting focus itself. The Entertainer background is also a natural choice for the character.
That’s it, that’s the essay. Anyway hot take: Hornet is an Oath of the Ancients paladin.
EDIT
I just realized that the Grimmchild could also come from a Pact of the Chain effect. Just reflavor a pseudodragon, ok bye