A Sybil attack is a type of attack on a computer network service in which an attacker subverts the service's reputation system by creating a large number of pseudonymous identities and uses them to gain a disproportionately large influence - wikipedia 
It is named after the subject of the book Sybil, a case study of a woman diagnosed with dissociative identity disorder. The name was suggested in or before 2002 by Brian Zill at Microsoft Research.
The term pseudospoofing had previously been coined by L. Detweiler on the Cypherpunks mailing list and used in the literature on peer-to-peer systems for the same class of attacks prior to 2002, but this term did not gain as much influence as "Sybil attack".
# Example
A notable Sybil attack in conjunction with a traffic confirmation attack was launched against the Tor (anonymity network) for several months in 2014.
There are other examples of Sybil attacks run against Tor network users. This includes the 2020 Bitcoin address rewrite attacks. The attacker controlled a quarter of all Tor exit relays and employed SSL stripping to downgrade secure connections and divert funds to the wallet of the threat actor known as BTCMITM20.
Another notable example is the 2017–2021 attack run by threat actor KAX17. This entity controlled over 900 malicious servers, primarily middle points, in an attempt to deanonymize Tor users.