the dresden files - How do death curses work?

This question contains spoilers from various books in the series.

In order for a death curse to have a long lasting effect, it has to be anchored to something. For example

Margaret LeFay had to anchor her spell to Harry via their blood relation to prevent Lord Raith from ever feeding again.

On the other hand, Quintus Cassius seems to have cast a long lasting death curse on Harry by cursing him to "Die alone". What did he anchor his spell to?

Has the spell been abated due to the fact that:

Harry (mostly) died in Ghost Story?

Finally,

Would Harry have been able to get a free death curse onto someone, such as Nicodemus had he had the foresight to set one up?

Answer

All energy setting up a death curse has to come from a living being, burns every last bit of energy that living being has out. The sacrifice of a life greatly powers the spell and can enable it great power, but typically in one big burst. Forethought and planning can structure the spell to do anything you want it to, so long as you can preserve the enchantment.

(marking spoilers for Changes and past)

For Quintis Cassius death curse, it wasn't having a continuing effect on Harry, just a delayed one. It would take very little energy for it to last for as long as it did, because it wasn't doing anything up until point of death (and in some arguments, not much even then). Even if that sustaining required more energy than Quintis life had put into it, it could leech from Harry, who couldn't defend against it latching on at the time. For Maggie Le Fay's curse, it was constantly keeping up the "starving" effect on the White King, or at least would have had to engage every time he tried to feed. That type of long term cursing wouldn't be able to sustain itself without help. She might've been able to use the King himself as a source, but because he'd already set up his magical immunities at this point that was out. Thus tying it to her bloodline, siphoning small amounts of energy over time to sustain itself.

For the second part of your question, Jim Butcher has gone on record and stated that

Quintis' curse has fired with Harry's death in Changes, as suggested by Harry hearing his voice over and over. The spell recognized it's opportunity and timeliness, and fired what energy it had left. There's no longer anything active to worry about there.

For the third part, setting up a "free" death curse is unlikely. The energy for the spell has to come from your life energy. If all of your life energy goes into a death curse, there isn't likely to be anything left for your ghost or spirit to build itself around.

Harry's "resurrection" consisted entirely of life support until his spirit was shunted back in, and that was the important part. Harry using a death curse would have destroyed him utterly before they ever got the chance to pull something like that.