Death curse

A death curse is a powerful magic curse which can be spoken by a wizard.

Description
A death curse utilizes all the power a wizard can access, including the energy it takes to stay alive, and so it is called a Death Curse because the wizard casting it will die by casting it.

The curse's target is often the wizard's killer, but it can apparently be directed at anyone, regardless of the target's involvement or nearby physical presence.

In the series
In Storm Front, Harry Dresden warns Bianca that if she were to kill him like she wants to right there and then, that his Death Curse would take her down with him.

In Death Masks, Harry uses his Death curse as a threat in an attempt to disuade Paolo Ortega from his duel challenge.

In Blood Rites, it's learned that Margaret Dresden cursed Lord Raith on her deathbed, preventing him from feeding, and thus prevented him from gaining strength.

Later in Blood Rites, Harry Dresden would use this knowledge to his advantage by causing Lord Raith to use up his reserves and getting Lord Raith to monologue about it while Lara Raith was hiding close enough nearby to hear him.

In Dead Beat, the necromancer Quintus Cassius, killed by Mouse on Harry Dresden's orders, hit Harry with his Death Curse. He wanted his curse to kill Harry, but he was too weak to manage that and so settled with cursing Harry to "die alone".

In Changes, Harry recalled Cassius' Death Curse when he was laid up in St. Mary's with a broken back an he was all aone in the dark feeling utterly helpless. Later, Harry used the threat of his Death Curse to destroy Chichen Itza in order to push the Red King into encouraging Arianna Ortega to answer Harry's challenge to a duel per the Code Duello.

Harry would be haunted by Cassius' voice in his mind telling him to "die alone" until the end of Changes, where a familiar voice tells it to "hush now".