Tatsumasa, the Dragon’s Fang

The hilt of this sizeable katana is crafted from lapis-lazuli, and inlaid with polished pearl carved in the shape of a twisting dragon. You can hear the faint hum of electricity coming from the sword, and when it strikes a target, lightning runs down the length of the blade. Legends are split on whether there is only one Tatsumasa or several, but each claims that the blade of the sword itself is made from one of the fangs of a mythical blue dragon, slain long ago. You gain a +3 bonus to attack and damage rolls made with this weapon, and attacks made with it deal an additional 2d6 lightning damage. Once per long rest, while holding the Tatsumasa, you can use your action to cause it to transform into a young blue dragon. Your DM has the statistics for the creature. The weapon remains in this form for 1 hour, or until it is reduced to 0 hit points or you use your action to cause it to change back. When the weapon returns to its original form, it appears in the center of the space previously occupied by the blue dragon. The blue dragon is friendly to you and your allies and obeys any commands you give to it. You can command the dragon using an action on your turn, otherwise, the dragon acts on its own, and attacks obvious threats to both you and it.