One of the living weapons: the Lunar Mercury...
There are others: Kagami's Organic Gold, a whip; Lagunahaan's Geo-Pyro Gate, a shotgun; Idou's Zenith Crio, a halbard; and Emperor Kura's Nox Light, a sword. But the one we're concerned with right now is Seeu's.
This weapon is in the shape of a scythe. It's viciously powerful; it seems to have the ability to rain down shards of metal on the battlefield. It has never been defeated due to this ability to effectively kill so many things at once.
"The rain of Lunar...they say it rained many times on the battlefields of the past. It didn't matter if you were friend or foe; after the rain, no one remained standing."
The Lunar Mercury is fiercely protective of Seeu; it won't even let other people touch him. It has this amazing ability to slice things in pieces; for example, in the first tankouban, Seeu slices Kaguya's house practically in half with just one swing of the Lunar Mercury.
These living weapons hold special powers; one of them is that their holders will not age. They are basically immortal unless they are killed by force.