Eldritch Symbolism. The attendant has advantage on Intelligence (Arcana), Intelligence (Investigation), and Wisdom (Perception) checks to notice, identify, or bypass magic writing and magic traps. It also has advantage on saving throws against magic traps and the effects of magic items. It can expend two Bardic Inspiration dice to cast glyph of warding without expending a spell slot as an action.

Spellcasting. The Yellow Sign Attendant is a 7th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 13, +5 to hit with spell attacks). The Yellow Sign Attendant has the following bard spells prepared:
Cantrips (at will): message, minor illusion, vicious mockery 
1st level (4 slots): disguise self, hideous laughter, sleep

 2nd level (3 slots): see invisibility, shatter, suggestion

3rd level (3 slots): fear, song of Hastur(SPCM), speak with dead

4th level (1 slot): greater invisibility, Yellow Sign(SPCM)

Turning Defiance. The attendant has advantage on saving throws against effects that turn undead.

Dagger. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Bardic Inspiration (3/Short or Long Rest). As a bonus action, the attendant can grant an ally that can hear it a d8 inspiration die if it doesn't already have one. Once, within 10 minutes, the target can roll the die after rolling an ability check, attack roll, or saving throw but before the DM declares success or failure. The result is added to the total.