Amulet of the Yellow Sign

An amber spiral with three crooked, queasy spokes rests on obsidian laid into a tarnished platinum amulet. It is a relic of a Great Old One -- Hastur, the King in Yellow. This amulet grants enormous power to those who use it and works tirelessly in Hastur's service.

Hastur's Favor. While a creature is attuned to the Amulet of the Yellow Sign, it gains +3 to its spell attack bonus and its spell save DC. While attuned, this amulet also functions as an Amulet of Health.

Horrific History. A creature of good alignment who closely inspects the amulet must make a DC 25 Wisdom saving throw. On a successful save, the creature takes 1d10 psychic damage and is stunned for 1 minute. On a failed save, the creature must roll on the short-term madness table and is frightened of the amulet until the completion of its next long rest; this rest will confer no benefits beyond HP restoration and the creature suffers horrible nightmares and a point of exhaustion. After a long rest, a creature that failed this saving throw is capable of attuning to the amulet and suffers no ill effects if it inspects it again. 

A creature of neutral alignment who closely inspects the amulet must make a DC 25 Wisdom saving throw. On a successful save, the creature is stunned for one minute. On a failed save, the creature takes 1d10 psychic damage and rolls on the short-term madness table.

Any creature who closely inspects the amulet experiences a thorough, rapid, and jumbled series of memories belonging to every being who has attuned to this item. These memories are not coherent enough to glean specific information from (e.g., as a substitute for interrogation), but do extend back to Hastur himself.

*Specific terms of confusion: a confused creature can't take reactions and rolls a d8 at the beginning of each of its turns. On a roll of 1 or 2, the creature cannot take an action; on a roll of 3 or 4, it makes a melee attack against a random creature within reach (if no creature is within reach, it can neither take an action nor move); on a roll of 5 or 6, it must dash and use all movement in a single random direction; on a roll of 7 or 8, it makes a melee attack against itself with advantage.

Harbinger of the King in Yellow. If the bearer of this amulet is targeted or affected by a spell cast by a creature of good alignment while it is astrally projected, the bearer can choose to absorb that spell into this amulet. These spells may be used by an evil creature attuned to the amulet's analogue in the original plane for up to 24 hours after the time of initial casting (using the bearer's spell attack and save DC). After 24 hours, these spells are "digested" by the amulet into charges -- one charge per spell slot level per spell absorbed by the amulet. When the amulet has collected 50 charges, the bearer can make a DC 25 Arcana or Religion check once per year to contact Hastur. When the amulet has collected 100 charges, the bearer can make this check once per month and with advantage. When the amulet has accumulated 500 charges, the bearer can make a DC 30 Arcana or Religion check once per decade to summon Hastur to the Prime Material Plane (this can only be attempted while on the Prime Material Plane). If Hastur is successfully summoned, the charges on the amulet reset to 0.

Corruption of the Yellow Sign.  Hastur's influence brings a powerful corruption that will eventually warp any conscience towards confusion and cruelty. Any creature attuned to Hastur's Yellow Sign with an alignment other than chaotic evil must make two separate alignment checks every day at twilight. Let n be the number of cumulative days the creature has spent attuned to the amulet. Roll percentage dice for each check and add the creature's Wisdom saving throw bonus: if the result is less than 100*(1 - 1/n), the creature fails the check and its alignment changes. If it fails the first check, its moral alignment moves from good to neutral or neutral to evil; if it fails the second, its ethical alignment moves from lawful to neutral or neutral to chaotic. If the creature is already of an evil or chaotic alignment, the corresponding check has no effect.

If a creature de-attunes from the amulet and had an alignment other than chaotic evil prior to attuning to the amulet, then it may attempt to retrieve some vestige of its former identity. Let m be the number of consecutive days the creature has spent without being attuned to the amulet. If the creature chooses, it may roll percentage dice once per day at dawn, adding its Wisdom saving throw bonus. If the roll is more than 100/m, the creature may choose to shift its alignment either further from evil or further from chaos.

Vessel of Hastur. Any attuned creature of chaotic evil alignment may be dominated by Hastur. Up to 3 times per day, Hastur may cast Dominate Monster (DC 25) on the creature at 9th level. If Hastur successfully dominates an attuned creature, that creature must make a DC 20 Charisma saving throw when the domination ends. After a failed save, any subsequent attempt to de-attune from the amulet will instantly kill the creature. Any creature killed in this way may only be revived by Resurrection, True Resurrection, or Wish.

Notes: Bonus: Spell Attacks, Bonus: Spell Save DC, Immunity: All, Set: Constitution Score, creature of neutral or evil alignment