Hack?

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. 

That is a Rogue Calendar account. 

You are using iOS 14 then Go to Settings ➔ Calendar ➔ Accounts and under Accounts see if an unknown calendar account is listed. If it is then tap on it and then Delete account.

Note that it is not a virus. It is something that got added to your calendar because you may have accepted an offer from a pop-up on a website unknowingly. Please be careful out there.

Axel F