The Spirit Pack is made up of spirits of deceased mistwolves. They run the Spirit Trail, a path that leads across the astral planes and to the Gates of Avalon (Adriane is one of the only mages to have the ability to walk the Spirit Trail). Their leader is a black mistwolf by the name of Chain.
Living mistwolves are able to tap into memories of the Spirit Pack. Since mistwolves have lived for thousands of years, this means mistwolves have millions of memories that have been passed on as each mistwolf dies.