Virus
Calender keeps on showing different events like keep your phone safe and different 18+ website links. I think of something like virus may have entered in the phone.
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That is a Rogue Calendar account.
If you are using iOS 13 then Go to Settings ➔ Passwords & Accounts and under Accounts see if an unknown calendar account is listed. If it is then tap on it and then Delete account.
If you are using iOS 14 the 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