The Solomon Keys are a pair of keys that, together, can open any door.
The Solomon Keys can change their shape to open any door. If someone steals them they will bring bad luck to them. They will turn against you if you use them to steal. One is long and golden. The second one is short and silver. The long, golden one can be used to open larger locks, while the short, silver one can be used to open smaller ones. One interesting result from this is that with just one of the Solomon Keys, one cannot open every lock in existence; one can only open half of the locks in existence.
When Cyrus Smith gets the Dragon's Tooth from Billy Bones, the keys come with it. He soon discovers their magical power and uses them many times. When new to Ashtown, he opens a burial with them but the resulting experience leaves him afraid. However, in The Empire of Bones, he opens more burials using them, in his and Antigone's search for the Brothers Below.