The enabling legislation is the Special Counsel Act of 1999.
But perhaps the question is specifically about the power to grant immunity, which would probably be an “inherent power” of a Special Counsel.
You claim this is an Act passed by Congress? Give me the citation to the US Code containing this act.
Wikipedia, for what it's worth, says there is no statutory basis for the current special counsel procedure.
With the expiration of the independent counsel authority in 1999, the Department of Justice under Attorney General Janet Reno promulgated regulations for the future appointment of special counsels. As of 2017, these regulations remain in effect as 28 CFR section 600.[5] While the regulations place limits on the authority of the attorney general, for example to fire the special counsel once appointed, they are internal Department of Justice regulations without an underlying statutory basis.