How can I model explosives with secondary effects?

Question

In my campaign, there is a crazy old shopkeeper NPC who has some arcane knowledge. In preparation to defend the town against an incoming zombie horde, the party paid him to make twenty one-pound small explosive barrels. Eighteen of those barrels were used to set a trap, and the party kept two. One of my players just decided to lob one into an oil fire in front of some charging enemies, and it happened to be convenient to end the session at the apex of the arc.

I am looking for the most RAW-conformant answers to two questions, considering the balance implications of having the primary effect (explosion) triggering a secondary effect (oil):

• How do I model the damage from the explosive? I want to treat this as thunder damage.
• How do I model the damage from the fire, both from entry into the hazard and from splashed flaming oil?

Answer 1

There is a model and statistics for several types of Explosives in the DMG on page 267-8.

The bombs are even specifically 1 lb.

Bomb: As an action, a character can light this bomb and throw it at a point up to 60 ft away. Each Creature within 5 ft of that point must succeed on a DC 12 Dexterity saving throw or take 3d6 fire damage.

Oil and catching fire, whether spread on a surface or covering a creature is actually covered in the PHB p 152, under the Oil entry.

Oil. Oil usually comes in a clay flask that holds 1 pint. As an action, you can splash the oil in this flask onto a creature within 5 feet o f you or throw it up to 20 feet, shattering it on impact. Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil. You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area. A creature can take this damage only once per turn.

This is statistics for 1 lb of oil. If there is more than that perhaps consider extending the duration of the burn. If there is enough to burn long enough perhaps consider doubling the damage or looking into the Fire rules.

One last thing, Improving Damage and Adjusting Areas of Effects on DMG P249, might be good to glance at. While it doesn't use specific examples for your situation it does provide some decent guidelines.

Answer 2

Use Fireball-like Stats

For explosives I usually just rule that it has the same size and damage as the fireball spell.

Each creature in a 20-foot-radius sphere must make a Dexterity save. A target takes 8d6 fire damage on a failed save, or half on a success.

I usually leave out the fire spread mechanic unless it’s an incendiary explosive. Damage, blast radius, and DEX save can be adjusted according to the explosive potency and size.