A death web (death spell) was a spell that could mimic a real death by making people believe that they have seen a death, and so make them viscerally accept the event as true.
Another death spell was cast on Kahlan Amnell to calm a crowd, and complete prophecy, by Zeddicus Zu'l Zorander. Richard Rahl, however, did not know about the event as he was in the Old World and was devastated to find that Kahlan was dead. When he visited her grave, however, the inscription caused him to realize she was not truly dead, and his former trainer Mord-Sith Denna came back to lead him to Kahlan in a place between the worlds.
An apparent contradiction exists in relation to the Death Spell. After Zedd casts the spell on a terrified Kahlan, shortly after having threatened to kill her himself, Zedd explains that the Death Spell cannot be cast successfully unless the target genuinely believes that they are about to die. In 'Debt of Bones', however, there was no indication that this was true. Zedd simply cast a spell on his daughter to help her sleep after being rescued from D'haran soldiers who had kidnapped her, and she awoke with the Death Spell on her. There was no indication given that Zedd's daughter believed she was about to die when the spell was cast, which suggests that Zedd might have been lying to Kahlan about the Death Spell's rules. While this is probably a simple continuity issue within the series, it is also possible that Zedd was play-acting for the benefit of the spell or punishing Kahlan for abandoning and manipulating Richard into travelling with the equally manipulative Sisters of the Light.
In Debt of Bones, Zedd says that when he was first shown his daughter's room he laid a few traps for those intent on harm. One of those traps killed Mariska when she came for his daughter. It was because his spell had killed her while she intended to harm his daughter that he was able to deceive the D'Harans using a vision of her killing his daughter. Perhaps the spell worked so as either the one who appears to die or the one who appears to kill believes that the killing will happen for real.