The Mark of Death is the most mysterious of all Dragonmarks. Originally, it belonged to the Necromantic House of Vol. A group of Aerenal elven nobles who experimented with Negative Energy in defiance of the races' usual ban on such practices.
Eventually, Erandis Vol, was born from the union with a Dragon and an Elf. It has been said that her Dragonmark was special in some way due to her Draconic blood. Nevertheless, she never got to use it because her house was then hunted down by both Dragons and elves.
The Mark of Death is the only Dragonmark that is believed to be extinct by the time of 980 in the Galifar calender. No member of the House of Vol survives that has not been either slain or turned into a member of the undead. The Mark of Death seems to have had some sort of power relating to the creation of undeath as Lady Vol was transformed into a Lich by her mother using her Dragonmark upon her.
What the Mark of Death is to have done or been capable of doing is, now, one of Eberron's great mysteries.
The Mark of Death was detailed in the The Lost Mark book trilogy by Matt Forbeck