User blog comment:KnocknickKnock/9 gates + master of the celestial realm/@comment-2.83.198.8-20131212210042/@comment-4829046-20131214041332

Or the Spirit King simply doesn't have a key. Maybe there never was one created since for one thing, Spirit King only appears in the Human World when Imposing the law, as stated by Loke so I don't think even the other CS mages have ever seen him outside of maybe some research books also perhaps humans just weren't able to control him before. Might change with Lucy though.