Hacked phone
I have had items added to my calendar saying that my phone is at risk of being hacked or has been hacked
iPhone 12 Pro, iOS 14
If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account.
If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account.