An adversary in a Sybil attack subverts a blockchain network after creating a large number of pseudonymous or forged node identities that allow it to gain disproportionately large influence. The attacker behaves correctly until it has a majority of nodes and then sabotages the network. A majority coalition might want to censor remaining nodes to insure that it received all of the transaction fees. The attack is named after Sybil, the subject of a case study of a woman diagnosed with multiple personality disorder.
PoS is more resistant to Sybil attacks than PoW because an anti-sybil mechanism is native to the Casper protocol itself: censorship is expensive. The feasibility of a Sybil attack depends on Casper’s slashing mechanism that cuts the stake from a bad node attempting to create separate identities or spoofed timestamps.