Rulebook: Hordes of the Abyss (p. 85)
You learn a language of truly dark power.
Prerequisite: INT 15, Will save bonus +5, Cha 15
There exists a language so dire, so inherently full of spite, malice, corruuption, and hatred that it is known as 'The Dark Speech'. The Dark Speech is extremely difficult to master. To actually communicate by means of the Dark Speech-that is for a knowledgable speaker to convey information to a knowledgable listener-the speaker must take great care, or both the listener and speaker will be harmed. There are no words in the Dark Speech for good concepts, such as kindness, mercy, and purity. However, evil characters can speak of misery, anguish, hate, and betrayal with an accuracy impossible in any other tongue. The Dark Speech has no written form. It cannot be transliterated into a written form without losing all its power and meaning. Besides communicating in the language of Evil itself, there are 4 ways a speaker can use The Dark Speech.
Benefit: You can use the Dark Speech to bring loathing and fear to others, to help cast evil spells and create evil magic items, and to weaken physical objects. The four basic uses are detailed below.
Dread: Whenever you use Dark Speech in this manner, you take ld4 points of Charisma damage, and every other creature in a 30-foot radius must attempt a Will save (DC 10 +1/2 your character level + your Cha modifier). The result of a failed save by a listener depends on the listener's character level and alignment, as detailed on the table below.
Level (Alignment) Result:
1st-4th (non-evil) Listener is shaken for 1d10 rounds and must flee from you until you are out of sight.
5th-10th (non-evil) Listener is shaken for 1d10 rounds.
11th+ (non-evil) Listener is filled with loathing for you but is not otherwise influenced.
1st-4th (evil) Listener cowers in fear for 1d10 rounds.
5th-10th (evil) Listener is charmed by you (as charm monster) for 1d10 rounds.
11th+ (evil) Listener is impressed, and you gain a +2 competence bonus on attempts to change his attitude in the future.
Power Whenever you use Dark Speech in this manner, you take 1d4 points of Charisma damage. By incorporating the Dark Speech into the verbal component of a spell, you increase its effective caster level by 1. By using it during the creation of an evil magic item, you increase its caster level by 1 without increasing the cost.
Corruption: As a full-round action, you can whisper vile words at an inanimate object and reduce its hardness by half. This use does not drain you, but you cannot use the ability more than once on a single object.
Dark Unity: You can use Dark Speech to establish a hive mind in any swarm of vermin or animals with an Intelligence score of 2 or lower. Thereafter, you can give the swarm one command as per the suggestion spell (caster level equals your Hit Dice). Whenever you infuse a swarm in this manner, you take ld4 points of Constitution damage.
Special: You gain a +4 circumstance bonus on saving throws made when someone uses the Dark Speech against you.
Normal: Attempting to utter a word of Dark Speech always ends in immediate death for a speaker who is not trained in its dark power. It is impossible to make someone use the Dark Speech if he is unwilling because the language's pronunciation is so exacting.