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The calendar has not been hacked. You have unwittingly subscribed to a calendar. See the information here.
If you go to Calendars at the bottom of the screen there, do you see a calendar listed that matches the color of these events? If you see a strange calendar there under Other or Subscribed, can you uncheck it? Then you may want to check, it seems they will add a strange mail account. Go to Settings>Passwords & Accounts and see if you find a strange email account there. It may be where the calendar syncing is coming from. Delete the account from there. You will also want to check Settings>General>Profiles, and see if there is anything unusual listed there.
See this, How to unsubscribe from calendars on your iPhone - Apple Support