When a creditor has discharged a debt you owed, you're generally required to include the discharged amount in your income.
That said, you may not have to include all of this canceled debt in your income. There are exceptions and exclusions, such as insolvency and bankruptcy. Please see IRS Publication 4681, Canceled Debts, Foreclosures, Repossessions, and Abandonments for guidance.
You might also consider reaching out to a tax professional, since you indicated the potential tax implications are significant.
You received Form 1099-C because your stock broker discharged a debt you owed, or an event occurred -- the negative equity write-off -- that either is or is deemed a discharge of a debt.
