How do the rules handle dropping an extremely heavy object from far up to create an explosive effect? [duplicate]

Question

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?

Answer 1

There aren't really any rules

There are no specific rules for falling objects in 5e, which comes back to D&D (5e) not doing physics well. It is left to the DM's intuition.

But that doesn't mean we have nothing to go on. The rules/guidelines on page 249 on improvising damage provide a rough scale for damage (number of d10s) for different kinds of 'events'. So let's try to see the one which is closest to your scenario.

The 'naive' answer here might be that a massive tungsten cylinder is equivalent to a flying fortress (apparently a more likely occurrence), the recommended damage for which is 18d10.

However the scenario you described is actually closer to Rods from God¹ which is, in short, dropping a massive tungsten rod from orbit producing the effect of a nuclear detonation without the bothersome irradiation afterwards. This might be considerable worse than a mere flying fortress. The last item on the improvised damage table (for 24d10) reads

Tumbling into a vortex of fire on the Elemental Plane of Fire, being crushed in the jaws of a godlike creature or a moon-sized monster

and if you feel being at the center of a nuclear-like explosion is equivalent to one of those descriptions, use that damage. If you feel it is worse, the follow up questions might be "how many dice do you have?", "how much time do you want to spend rolling them?", and "is the outcome going to be anything other than they are dead?".

Of course this is only the damage magnitude of the effect. The radius of effect and whether creatures (bbeg and key minions) are affected is going to be up to you (the DM) and maybe in particular how much you are willing to let creative solutions solve/bypass challenges. There is no set right way to solve this, but personally I would let this "solve" the problem of all the minions (meaning they don't have to be fought), leaving only the 'final boss-fight' against the bbeg (assuming this is your story structure).² The main argument I have for doing so (in addition to "I had planned this fight to be really cool") comes down to having all in the party participate: If the powers (i.e. spells) of only one party member solves the problem/conflict, it means the other members aren't contributing. This can very easily lead to them feeling redundant or useless, meaning they aren't having fun. And the DMs most important job is to try to make everyone have fun.

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1: We are here using the real world idea, not the fancy sticks handed out by celestial beings as is more common in D&D.

2: We are leaving the bbeg alive under arguments that their stronghold is sufficiently fortified, quite possibly magically. Remember that bad guys tend to have defences that often parallel or outperform the good guys attacks in heroic storytelling.