set on/uponsomeone
phrasal verbwithsetverbuk/set/us/set/present participlesetting|past tense and past participleset
[often passive]
toattacksomeone:
He was set upon by aviciousdog.
tosurroundorcatchsomeone andpreventthem fromescaping:
As heleftthetheatre, thesingerwas set upon byfansdesperateforautographs.