How do I add a percent modifier as a separate damage formula?

Help
1

I have a weapon that has a “plus one half more fire damage” modifier. I want to configure 1d12 + @mod + 3 [slashing] damage, plus (slashing)*0.5 [fire] damage as a separate damage formula. How do I reference the result of one kind of damage role from another damage role?

2

This would be a better question to ask on the foundry discord as the creators and mods of foundry are active and able to provide the best case answers. Exactly how you would go about adding or referencing 1 roll to affect another is beyond the scope of foundry questions the mods of forge could typically answer. Foundry VTT

3

The answer might be specific to the system you’re running and which modules you have, but assuming that you’re using only a system like dnd5e, you can’t directly reference the roll amount of a previous inline roll with the damage formula, but you could do what you’re describing with a simple macro.

The folks over in the Foundry Discord should indeed be able to help you out with this, but if not, you could work off of and refine this one:

// Assuming you're using v11.315 and dnd5e v2.4.1
// Put your own actor's ID in here
let actor = game.actors.get("zyfXujEdnZI38xCV")

// Define the modifier (@mod)
let mod = actor.system.abilities.str.mod; // Assuming strength modifier, change accordingly

// Roll the slashing damage
let slashingRoll = await new Roll("1d12 + @mod + 3", { mod: mod }).roll();

// Calculate the fire damage (half of the slashing damage)
let fireDamage = Math.floor(slashingRoll.total * 0.5);

// Create a message for the chat with the results
let messageContent = `
  <strong>Slashing Damage:</strong> ${slashingRoll.total} <br>
  <strong>Fire Damage:</strong> ${fireDamage}
`;

// Send the message to the chat
ChatMessage.create({
  speaker: ChatMessage.getSpeaker({actor}),
  content: messageContent
});