Given how they have been described in the books, I would say that any practitioner could *in theory* cast a death curse, however, *in practice* it likely doesn't make much sense to focus on anything but a wizard or above's death curse.

A death curse is a lasting magical effect that either has to be sustained by the sheer amount of power thrown at that last instant of death, or has to be tied to something else to keep the curse powered. Remember the curse has to last over the person day in and day out past all of these sun rises and the water that flows over them and whatnot. I imagine besides going ka-blewy, the only effective way to cast a death curse is to tie the spell to something else to keep it renewed. (Like Maragret's death curse was tied to her children.) Anyone who isn't a wizard likely doesn't have enough power or training to realistically create a death curse worth mentioning. They might have enough power to throw out a spell of vast proportions for them, but given what a wizard likely slings about, it's not something comparable or noticeable.

Also something to keep in mind is free will. I would argue that only a practitioner who has free will can cast a death curse. We get into a grey area with vampires as they seem to be creatures with far more free will than other monsters, but less than humans. Vampires aside though, most monsters shouldn't get a death curse. Faeries, old gods, dragons and the like are all bound by rules and limitation that might not be readily apparent, but forbid them from taking actions of true free will and thus cannot act outside of those restrictions. further, it is entirely possible that most of the power they wield are mantles of one form or another and the power of a mantle can't simply be used up and discarded, it must be passed to another individual (as logic would dictate by the very nature of a mantle of power).

Or maybe you die anyway if you use your death curse.

It is basically a spell that uses all your live energy cast by a wizard just before he is going to die using up all his live energy. If you do it to early and your enemy does not kill you you still die. That is why you should cast your death curse not too late but also not too early.

I'm pretty sure this is the case. The way death curses are described it seems like a last ditch effort to use up all the energy in your body you have left to cast one final spell in order to harm/protect/whatever at your target. While the wizard might have been effectively killed by whomever put them in the situation where they are using the death curse, it's always seemed like more of a final suicide option. The wizard, understanding they are doomed anyway, takes the reigns and says "I'm going out on my own terms".

Harry does almost kill himself with magic a few times in the book, just when he is slinging it around at his enemies. It's entirely possible if you don't stop yourself in time you could bleed yourself dry with magic, possibly with one big spell.

As far as the Outer Gates and Outsiders go, I will reiterate what the others have said: Cold Days definitely has a lot of information about both.

But if you're looking for additional information, definitely go to Lovecraft for inspiration (as that's what Butcher is drawing inspiration from). TV Topes has a couple of great articles that could help ge tthe jiuces flowing for this sort of thing.
http://tvtropes.org/pmwiki/pmwiki.php/Main/EldritchAbomination
http://tvtropes.org/pmwiki/pmwiki.php/Main/CosmicHorrorStory

Thank you, Haru and Sanctaphrax. Those are definitely great ideas.

Basically, the ritual would then allow the properties of the sacrifice be distilled in its purest form, so the sorcerer can digest them. Of course, that could have side effects similar to summoning a demon, but kind of subvert the trope. If he sacrifices an aeromancer on a stormy cliff, the wind itself might howl in protest and come after him. Siphoning the werewolf out of the sacrifice might also siphon the human out of him and go into the nearest animal, creating a wolf-were with the memory of what happened to him.

I like the concept of an 'innate backfiring' as it were due to the nature of the ritual, so that it always results in some sort of twisted  echoes of the original power stolen. It's a great way to sort of ominously have nature back-lashing against the inherent perversion of magic.

Narratively speaking, I suggest having him harvest power from the cult that taught him his magic. Have them perform religious ceremonies to empower his rituals.

The idea of drawing from cultists is good. Maybe I'll have him (and the Red Court Vamp who now holds his leash) steal a few of the cultists and/or recruit his own. I want to try to keep the Cult he learned this from a secret and have them remain in the shadows for a little while. (This will be the first scenario for my players, so i want to build to that reveal a little.)

Mechanically speaking, human sacrifices can provide all of the shifts you'll ever need.

While technically that's true, I do want to make sure that players get a grasp for just how much power needs to go into a ritual needed to permanently steal another creature's powers. Human sacrifice is a good base, but I feel like other elements might be needed to drive the idea home.

If anyone else has any ideas, feel free to continue to chime in. Ever if I don't use the ideas for this particular scenario/situation, it might help for later ones.

Well if it's going to be late 1940's-1950's then maybe you could tie in the aftermath of WWII in some way. Are your PCs going to potentially be globe-trotting or are they going to be locked into New York?

Some ideas:
Former Nazi scientist who was working with Outsider magic tries to spring a ritual on the city (could tie in to your horror/noir theme if you were thinking of Call of Cthulu-esque escapades). This could also give your PCs a hint as to who the real Big Bad is.

House Malvora tries to orchestrate a second Great Depression following the war to generate more Despair, feed, and increase their power while making a move on House Raith for supremacy.

Also how do you plan on incorporating the Native American aspects into it?

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Naagloshi tend to only roam on sacred ceremonial ground and NYC doesn't seem to have a lot of that. Are the Wendigo going to be conscious or just mindless eating machines? I think if you're going the Wendigo route working in some Ghouls into the cult would make perfect sense. Maybe at some point a Ghoul clan makes an assault on the city to spread Terror (one of the Gods bailiwicks) while the cult does something to progress the ritual.

If you're trying to make a mystery/noir game then working in "mysterious disappearances" seems to be a safe way to get the mystery going and make the PCs get invested in figuring out What Lies Beneath. Maybe the cult is kidnapping people? Maybe ghouls are kidnapping people? Maybe a rift is opening up/being opened up in the NeverNever that is close to The Outside.

Wow these are all really great ideas, thank you. I'll probably incorporate the idea of Nazi's tapping into Outsider power, because that is just a great image in my head. And the idea for House Malvora is great too, because I wasn't sure how I was going to work them in (as one of my players is going to be playing a member of House Malvora).

As for working WW2 in, I've been trying to find ways to do so without it overtaking the tone of my game, but first section of the campaign is set during late WW2. It'll give the city this sort of tense and almost abandoned feel, because all the mortals are distracted by a war an ocean away. The monsters are left far more unchecked than before. But I figure it'll be a mixture of the game is set primary in New York, but they might have the occasional adventure outside the city.

As for the Native American aspects,

If anyone has any cool ideas for a game set in the 1940s as well, i'd love to hear them. I've got a bunch of stuff figured out for my game, but new ideas always can always help make things cooler.

Thank you all for the advice. I'll keep in mind the idea of limiting the number of Aspects and such people take.