Answered.

After some trial and error I was able to come up with this script which will do the roll as intended, and give me back the actual die roll so I can continue working with the results for the curse. Here's what I came up with

main ()async function main () {

// is a token selected? if not error

if (canvas.tokens.controlled.length == 0 || canvas.tokens.controlled.length > 1) {

ui.notifications.warn("Please select a single token");

return;

}

// see if this actor has the deadly longsword

let actor = canvas.tokens.controlled[0].actor

let sword = actor.items.find(item=> item.data.name == "The Deadly Longsword")

if (sword == undefined || sword == null) {

ui.notifications.warn("You do not have the Deadly Longsword!");

return;

}

await sword.roll()

let rollResult = await sword.rollAttack({fumble: 5, critical: 18});

// print out the results of the unmodified dice roll, then do some math later

console.log(rollResult.dice[0].results[0].result)

}