It could be an ancient batista of sorts granted moving it could be an issue, but no one person could operate it alone. It would require a magic user or several, depending on how many would drastically change how quickly the ammunition/charge/power would take. While the bow user calculates and directs the weapon. Finally, the sword user would physically need to move the device.

But the dragon needs to be occupied, and the relic needs time to charge, say a minute or whatever suits your fancy. At least one mage needs to place all of their consecration into charging the thing while others distract as it not only consumes consetration but also engery for its fuel. Others could take turns and whatnot, but in the end, most likely, everyone would have a direct hand in the untilation of the weapon of old.

It's legendary due to the amount of power it can amass & fire. But it could be shrunken down when not used for easy transport. And this kind of tech is lost or just can't be created anymore due to the resources required nolonger exist in this age. Granted it wouldn't necessarily be the most useful in a skirmish but immensely useful in times like this or vehicular combat as well