On his head a cap of black gauze; about his waist a belt of rhinoceros horn.

The portrait opens with the two things a Tang official was known by across a courtyard: 烏紗, the black gauze cap of office, and a belt of 犀角, rhinoceros horn — properly a 犀角帶, which only a senior minister could wear. The judge of the dead is dressed as a civil servant.

The black gauze on his head trails soft streamers; the horn about his waist gleams with gold inlay.

The same two items again, now with the trimmings. 金廂, jīn xiāng, is gold inlay: here 廂 stands in for 鑲, a swap of one same-sounding character for another that Ming printers made freely.

In his hands he lifts an ivory tablet wreathed in auspicious mist; on his body a gauze robe half-veils an auspicious glow.

The 牙笏, yá hù, is the ivory tablet an official held at his chest in audience — part notepad, part badge of rank, and a reminder that he is not meeting your eye. 著 here is zhuó, "wears." The 羅袍 is a robe of fine silk gauze.

On his feet a pair of white-soled boots that mount the clouds and hasten the mist;

粉底靴, white-soled boots, are formal court dress and nothing stranger. What is strange is what they do next: 登雲促霧, mounting cloud and driving mist. The costume is a bureaucrat's; the mileage is a god's.

in his bosom he tucks a single register of life and death, wherein survival and perishing stand fixed.

And here is the one detail that gives the game away. Tucked in his robe is the 生死簿, shēng sǐ bù, the register of allotted lifespans kept in the courts of the dead. Its entries 注定 — "fix by writing down" — what happens to you. Remember the object: before this chapter is done, somebody alters it with a brush.

His temple-locks, loose and unkempt, flutter above his ears; his beard, streaming and dancing, curls about his cheeks.

Loose hair and a flying beard, standard flourishes in this kind of portrait, giving the figure a wild, not-quite-alive energy the court dress alone would not carry.

In days gone by he served as a minister of the Tang; now he keeps the rolls in attendance upon King Yama.

The couplet is the punchline: it tells you who he is. 為 here is wéi, "served as," and 相 is xiàng, "minister." 掌案 means keeper of the case-files, and 閻王 is Yánwáng, Yama, the sovereign of the dead. So: 崔珏, Cuī Jué, once a minister of the Tang, now a magistrate in hell — which makes him, from the emperor's point of view, the most useful man in the underworld.