Purple studded leather armour
When Mira completes the puzzle set by Farlu Naenreh in the catacomb beneath the Spire of Illusion, it opens a small chamber at the south end of the room. Inside this is a golden key that Mira pockets, as well as a set of studded leather armour worn by a wooden mannequin.
The armour is finely crafted from dark leather, with lots of organic curving shapes. It is dyed purple, and has small elements of silver running through it. When Mira finds it, it has a fine gold chain in one of its pockets.
Mira takes the armour from the mannequin and wears it underneath his normal robes, finding that it is warm with magic. Once he's wearing it, he pulls on the gold chain, which is attached to a kind of golden locket lacking a clasp. Suithrathus Luirmo later identifies this as the Seal of Illusion.
The mannequin that wore the armour when Mira found it is attached to a bust of his own face (at that time, gender-swapped by the mirror device beneath the Spire of Transmutation) with a smug grin. Before Mira leaves, he gives the bust a thumbs up, but as he turns to leave, he is convinced the bust winks at him.