The Eye of Zed

You spend the casting time tracing arcane sigils alongside abyssal runes in the shape of an eye, drawn with the blood of an arcane spellcaster capable of casting 6th-level spells or higher. The symbol must be encrusted with residium (a resource meant to represent whatever substance passes as a magical catalyst, reagent, or fuel in your campaign, here it is called residium). You adhere this symbol to a demon of Type 5 or lower with an Intelligence score of 19 or less.

The target demon undergoes instillment (the process by which a Demon Lord or greater fiend alters lesser demons to reflect their own nature and themes). It gains +3 to Intelligence, to a maximum of 20. The demon also learns one 1st-level spell and one 2nd-level spell that you know. These spells function as innate spellcasting using Intelligence as the casting ability.

The demon gains proficiency in Investigation and learns one language you know.

The symbol manifests as a serpentine eye wreathed in ethereal purple and green flames, usually placed on the forehead as a third eye, but it may be placed elsewhere. This eye is dormant unless activated by a bonus action, which grants the demon the constant effect of Detect Magic while active.

While under the Eye’s effect, the demon's thought processes become more analytical and make it more likely to use critical thinking. Persuasion checks that rely on deductive reasoning and logical arguments against the demon are made with advantage, provided they do not conflict with the demon’s goals, inclinations, or personality. This spell does not charm the demon, and its chaotic, violent nature remains, although somewhat tempered, but still intact.

The effects are permanent unless overwritten by a similar effect or another instillment from a Demon Lord.