Shadow counsel or a shadow lawyer is a term used in law to mean an appointed, duplicate, lawyer as an auxiliary or alternate, should the original lawyer (or legal team) fail in some way.
Shadow counsel is a legal term referring to a second lawyer that is appointed in secrecy to protect a witness from the other defendants who may be sharing the same lawyer (and thus, the known appointed lawyer is biased.) Such a situation could be a drug conspiracy or people forced into being accomplices by a mob leader. Shadow counsel advises in the best interests of the witness.
A fictional, yet informative example would be the Law & Order: SVU episode, "Ace."
The term is also used in other contexts with somewhat different meaning. For instance, conspiracy theorists use "shadow counsel" to refer to the secret police or any sort of hidden persons with power.