The Yellow Sign is a sign connected to Great Old One Hastur, the Unspeakable One. It is sometimes said that the sign is useless until the arrival of the King in Yellow into our world.

This sign is the major symbol of the cult of Hastur, also known as Brothers of the Yellow Sign. BOTYS is a cult of worshippers of Hastur which has connections to the King in Yellow. The Brothers congregate to destroy the outposts of the fungi from Yuggoth, as well as to torture members of the Great Race of the Yith for information about other times and places.

The King in Yellow is play written in the late 19th century and is also the name for an avatar of Hastur or possibly Nyarlathotep, which is the title character in of this play. The King takes the form of a gigantic human dressed in tattered yellow robes and sometimes wearing the Pallid Mask (of Truth).

To give you an example of the dangers of the Yellow Sign here is some information on what happened to P cell who got their hands on the book:

The agents of P cell confiscated the book from a professor who was very puzzled about it. Another cell was dispatched after P cell hadn't reported in for three days. They found one agent of P cell waiting in the safe-house, sitting in a chair without a shirt, yellow sign painted on his chest (don't ask me, why!) revolver in his right hand, with the dead bodies of his two fellow Delta Green agents lying on the motelfloor. He was waiting for en entity he called the Keeper. The other cell had no choice but to contain the agent and call ANDREA.

The Keeper of the Yellow Sign

The Keeper usually possesses the body of a large, recently dead human to complete its loathsome task. The Keeper will destroy in order all those who haven't made the Unspeakable Promise and have possessed the Yellow Sign more than one hour. Keeper is a creature of stars and is not bound by earthly limitations, so material barriers cause it no discomfort. All weapons seem to be quite ineffective agains the keeper, but it is said that: "...The Keeper desireth two things, the Signe and the Propaner himself, though a portion will satisfy his Hunger...".

Powers of the Sign (for simulation purposes):

It may only be used by those who have made the Unspeakable Promise, in which case it provides a 5 to 10 % bonus in all incantations dealing with Hastur and his minions. Such items are only given to those of the highest power in the Great Old One's cult.

For those who have not made the Promise, the amulet has a much different effect. First it will summon the Keeper of the Yellow Sign to retrieve the Sign and destroy its owner. The other result takes effect when the character interacts with others. He or she now seems to see the base emotions - such as anger and jealousy, and lust - all of those around him or her. At first these are only glimpses, but later these feelings are clearly reflected in their faces all the time. The investigator will lose 0/1D2 SAN each day that he sees even one human being other than himself. And of course the Sign will warp its possessors dreams, and give him visions of the city of Carcosa and the Lake of Hali.