Blood lock

According to the Fifth Doctor, a blood lock was "one of the strongest bonds in the universe". It required a specific type of blood to open it. The Osirians used such a lock on Sekhmet, to keep her imprisoned on Peladon. To open it, four royal women had to give their blood. It was for this requirement that Queen Elspera and Ambassador Alixlyr of the Martian Empire were murdered. (AUDIO: The Bride of Peladon)