The Dragonstorm Blade is a ruby and obsidian blade, and the main weapon of the Lost Legionaire Rubinia. It has so far only appeared in Aetheria Offline.
How Rubinia obtained the sword is unknown as of yet, but she either crafted it, found it somewhere, or it was given to her by Herobrine following her decision to work for him. She took it with her when she arrived in Villagetown, pretending to be noble. However, the sword most likely has aggressive enchantments which only a Nethermancer and traitor like her would use.
The Dragonstorm Blade, along with the Moonshine Sabers, Critbringer, and Nayte’s bow (I can’t remember the name), is one of the few weapons so far in the series belonging to a Legionaire to be named.