The situation is as follows:
Boris is a 13th level Illusion Wizard. Boris and his party have proximity to BBEG's hideout city and want to do as much damage as possible to both the city (buildings & infrastructure) and its inhabitants. They have a plan:
They have a method for reaching a height of up to 1km above ground level (whether it be an airship or repeated casts of the Fly spell), and Boris has a plan for eviscerating the BBEG's evil city. He flies up 1km above the city, and begins casting Creation at 7th level to create a 25ft long pointed Tungsten rod in mid air over the city. Tungsten, as per (somewhat rough) 5e rules, should be classed as a non-precious metal - so the rod lasts for a total of 12 hours.
Immediately after being cast, it should begin falling towards the city, and quickly reach terminal velocity. Moments before it hits ground, Boris will use his 6th level feature Malleable Illusions to change the form of the Rod to a 3375 ft3 cube of pure tungsten. The cube has a mass of ~ 1844.5 Metric tonnes (1,844,488.46 kg or 4,066,400.99 lbs for reference), and impacts directly in the centre of the city. In the real world, if an object such as that had enough mass & velocity, it could generate explosive force equivalent to an atomic explosion.
Assuming any direct reference to how this would function in real life, some form of explosion (AOE damage) would be caused.
According to the rules what happens if I do this? What effects does it cause?
What rules exist that would relate to this scenario - specifically - to the idea that simply dropping an extremely heavy object would displace (with explosive force) a large area of mass at its impact?