What is the Yunium Key for? I couldn't but it from Steamworks yet so I made one in Gentech. So now that I have it, what is it good for if anything?

It is to make a steamwork pet. ^_^

Edit: Zira city you go to :

https://subeta.net/explore/masquerade_schematic.php Schematic Shop: it's only opened 8 AM to 8 PM. The blue prints are 1 mil sp each.
and Full Steam Ahead https://subeta.net/explore/masquerade_workshop.php

You use the key and blue prints at Full Steam Ahead, choose your pet and in three give it 4 days, you get a new color pet :)

Ah..I see. Thank you for the help :)