Does it have to be brutal? Does it have to lots of death?
Does there have to be a war?
Do the characters have to lose family members?
Do the characters have to murder someone?
Do the characters have to be starving or struggling in some way?
Does there have to be lots of night-time scenes and very little laughter?
Or can none of these things happen and still somehow be dark fantasy?
If yes, then how?
I think you could pull off happy if you Morticia/Gomez the whole thing, where pain and suffering is just demented love. No death required.
On a more serious tone, Interview With a Vampire twisted to a more fairytale style would work.
What we have a problem with is placing something like Save The Cat in this asthetic. That death beat is really death, and it has to be more disparing than the rest.
My suggestion would ne INVERT the sucker, right there. Gloom and no hope is the asthetic of this thing. Up the stakes by giving these characters a chance for a future suddenly dangled in front of their face. All of sudden LIFE is an option. The stakes become real, then.
So, gloom and no hope is the aesthetic, but there can still be a chance of hope? But maybe said victory or a chance at life is only obtained after devastation? Like, the world can only be saved if millions die? Or could it be more personal to the character. MC can save her species, but only when her family dies?
According to Wiki, dark fantasy is fantasy that has elements of horror added, and on Goodreads the dark fantasy lists usually include books like The Hobbit, Coraline and The Ocean at the End of the Lane, so I’d say happy endings are fine as long as the tone is dark.