Grave callers apparently belong to an order of assassin-bards that basically function as normal assassins, except that they probably listen to The Cure and Nirvana while doing the rounds. Its a sound concept, I suppose, and will probably work out well if your group is tired of the same-old boring assassins that do things the old fashioned way (durn kids and their music).
Not much is said about the "shadowy organization henceforth referred to as the Grave Callers," but I dont think its necessary since it operates much as I would expect any other order of super secret assassins to: they single out people to kill and then forward you the instructions. The only difference is that the required class is bard instead of rogue.
The key feature of the grave caller is the "deathmark", which can be used once per encounter and lasts for the entire duration. By itself it allows you to deal damage to an affected creature even if you miss, but your grave caller spells only work on a creature affected by your deathmark so its kind of a required opening act.
Nothing really comes out and grabs me. Its standard fare, good if you wanted to play a bard that kills people for reasons other than pure personal gain. Perhaps if you wanted to play an assassin of another tune? Maybe they should have called it a deathnote instead...
Okay, I'm done.