Virus
I got virus on my iPhone, every 10 minutes I get new event in my calendar with the warning that there’s virus! It is impossible to remove created event
iPhone 11 Pro Max
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.