|"Are you a wizard or not?"
- Albus Dumbledore: "Oh, surely not. So crude."
- Harry Potter: "What is it, Professor?"
- Albus Dumbledore: "I rather think that we are required to make payment to pass. [...] Blood, if I am not much mistaken. [...] The idea, as I am sure you will have gathered, is that your enemy must weaken him or herself to enter. Once again, Lord Voldemort fails to grasp that there are much more terrible things than physical injury."
- — Albus Dumbledore explains Voldemort's protection on the Crystal Cave to Harry Potter.[src]
Weakness payment was the way of gaining entry to the inner part of the Crystal Cave. It consisted of weakening the person who wanted to gain entry in any possible way, and was created by Tom Riddle in his early teenage years in the 1930s.
In 1979 Regulus Black had begun to have doubts about serving as a Death Eater, but was reluctant to act. One day, Voldemort asked Regulus for the use of his house-elf, Kreacher. Voldemort used Kreacher to test the defences around his locket Horcrux, leaving him to die afterwards. Kreacher was able to escape using house-elf magic, and told Regulus of what had happened. This led to Regulus' defection and he travelled to the Cave to replace the locket with a replica. He had to get past the passage which required that he weakened himself.
In 1997 Albus Dumbledore and Harry Potter Apparated to this cave to break into the place where the Horcrux is hidden. In order to get inside, Albus Dumbledore used his knife to cut his hand open and use his own blood to get past the passage and into the inner part.