The Rainbow Undecimber (무지개빛 십삼월, 무지개빛 十三月, Mujigaebit Shipsamwol; "Rainbow-Coloured Thirteenth Moon") is one of the 13 Month Series created by legendary craftsmaster Ashul Edwaru for the Princesses. It has never been granted to a Princess. Unlike most of the weapons that form the 13 Month Series, the Rainbow Undecimber is an Arms Inventory. 
Notes and Trivia
Contrary to what some may say, the word "Undecimber" is correctly spelt with an "i", not an "e". This because it doesn't entirely reference December. In calendar systems that have twelve months, a thirteenth month, were it to exist, would be called "Undecimber".
It is the only 13 Month Series not awarded to a Princess yet. It still lies in the possession of the higher ups.
It is unknown if it is an Arms Inventory since the translation stated it's an Inventory and nothing else. There are multiple types of Inventory, such as Armor Inventory and Reel Inventory.