It’s not malware or anything notable, it’s a subscribed calendar.
This calendar was offered by some website, and was accepted.
This calendar is clearly filled with advertising, and as that’s no longer desired (if it ever was), delete the calendar subscription.
Tap the Calendar button at the bottom of the Calendar app main display, scroll down and find the subscribed calendar block with the subscription that matches the color of the events that have been showing, tap the information ℹ️ button to the right of that matching subscribed calendar, scroll down, and delete it.
As correctly mentioned above, Settings can also be used too.
There’s no malware here, and nothing else needs to be addressed.