Shadowtongue

Often revered as a foul and putrid language, it is often spoken by undead creatures in varying degrees of proficiency. Most of awakened undead have a natural grip on the language and sometimes fall back into it, when in a state of enormous stress, or hatred. The language itself consists of sharp, unnatural tones, a deep guttural tone interrupted, by strange hisses and shrieks.
The is also a dark alphabet wich is a series of symbols resembling various scratches, bitemarks, eyes and bones. Though it sounds savage, some undead elevate this alphabet to a macabre art.
Some necromancers call it the voice of souls and learn the language specifically to communicate with the dead to gain intel.
Most of the heard language will most likely be slivers of it grunted by mindless ghuls, or shrieked, by undead mages, without their own mind, acting on base instincts.