3getting it
It comes out of Hardmode treasure bags with the rest of the matching developer set, so you need Expert or higher and a boss that actually drops a bag with a set attached. Queen Slime is the exception and never gives it. If you have already cleared the bosses on a world, the remaining route is the Dye Trader, who can hand it over as a Strange Plant reward once the Moon Lord is down.
how it looks
It stains the character a dark purple and lays a faint shimmering wash over the body. It reads much better on plain armor than on anything already busy with color, and it is close enough to Phase Dye that you can mix the two in a set without the seams showing.
trivia
This is the only piece of developer kit obtainable in a Classic world. It is also the only dye in the game whose name never says dye.