Hacked calendar
I have checked other inquiries and I do not see any rogue calendars to delete. I have ios14.4.2
Question marked as
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.
When checking “Accounts”, check for any subscribed calendars.
The Apple support article is below.