I believe that the Ancient Weapon Uranus is a Devil Fruit that grants whoever eats it the ability to control the weather. Gol D. Roger possessed this devil fruit and after his execution, Dragon ate the fruit and gained its power.
Evidence that Uranus is a Weather Controlling Devil Fruit
Uranus being a weather-controlling Devil Fruit would fit into the themes of the weapons being related to the gods they were named after, as Uranus is the God of the Sky.
Shiki discovered that Roger had discovered an ancient weapon, and wanted to team up with him to take over the world. This weapon could have been Uranus, with Shiki only knowing that Roger had it, but not what it was or what the extent of its capabilities was.
Evidence That Roger Had Uranus
During the Battle at Edd War between Roger’s and Shiki’s forces, it was said that a giant hurricane engulfed the area, decimating Shiki’s fleet and leading to Roger’s victory
The chances of a hurricane appearing and only destroying Shiki’s fleet while leaving the Roger Pirates unharmed is practically zero. However, it would make a lot more sense if Roger used the power of Uranus to summon and control the hurricane to destroy Shiki’s forces
Evidence That Dragon Ate The Fruit After Roger
During the Punk Hazard Arc, it was revealed that when a Devil Fruit user dies, their Devil Fruit is reincarnated into a nearby fruit
Dragon could have known this, along with the fact that Roger had eaten Uranus.
It was shown that he was present at Roger’s execution. He could have easily hidden a fruit under his cloak, in which Uranus would have been reborn into, allowing Dragon to eat it and gain its powers.
Evidence Of Dragon Using Its Power
It has been hinted at several times in the manga that Dragon possesses some sort of ability that lets him manipulate the weather, as there were several abnormal weather events at places where he was present.
While Dragon was at Loguetown, a bolt of lightning saved Luffy from Buggy. A blast of wind also saved Luffy and his crew from Smoker and his marines.
During Ivankov’s flashback, it was shown that Dragon often looks towards the East Blue, with the wind blowing in the same direction. Ivankov believes this is because Dragon’s family(Luffy), lives in the East Blue. If this is true, then Dragon could have been unconsciously using his ability to cause the winds to blow in the direction of the East Blue.
A blast of wind created a tunnel in the fire at Grey Terminal in order to allow its inhabitants to escape to Dragon’s ship.
Finally, if Uranus really was a weather-controlling Devil Fruit, and the World Government knew that both Gol D. Roger and Dragon had eaten it, then that could be part of the reason they were both so wanted.
*Theory by strongerthenbefore20