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.
Least, Lesser, Greater, and Siberys Marks of Death
Eventually, Erandis Vol was born from the union between a Dragon and an Elf. It has been said by Keith Baker 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 calendar. 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.
Behind the Scenes
The Mark of Death is one of the few "permanent mysteries" of Eberron in that Keith Baker has stated that the truth of its nature will never be revealed in an official supplement. Keith has stated that the true nature of the Mark of Death, like that of the Day of Mourning, is for the Dungeon Master to decide. In his opinion it would not kill, because Dragonmarks are almost universally positive/constructive in their effects: hospitality, creation, taming, etc. As such, the Mark of Death would do things more along the lines of speaking with the deceased, raising undead and controlling them (as well as those previously reanimated), or setting undead to rest. He also stated that, in his view, the Dragonmark that Lady Vol possessed was not a Siberys Mark, but even more powerful because of her draconic heritage. Essentially, she is a living Eldritch Machine.
The Mark of Death (or an abberant Mark with death-related powers) was detailed in the The Lost Mark book trilogy by Matt Forbeck
References
Baker, Keith (18 April 2012). Dragonmark 4/18: The Mark of Death. Retrieved on 9 September 2013.