Definition of "in league"

Sarah Purfoy was in league with the convicts--was herself the wife or mistress of one of them. She had come on board armed with a plot for his release, and this plot was about to be put in execution.

1874, Marcus Clarke, For the Term of His Natural Life Chapter VII

I'm in league with Satan / I was raised in hell / I walk the streets of Salem / Amongst the living dead

1981, “In League with Satan”, in Welcome to Hell, performed by Venom