A “reversal” is the term applied to an unusual but always fatal failure to claim a Sundered One. Note: “reversal” only applies to the initial claiming attempt. Sundered Ones have been known to “slip their leash” later on, but in that case, the human usually survives.
When a human claims a Sundered One, the first few moments are the most crucial; if the human fails to make that connection properly, the Sundered One can push back, in essence reversing the process… but the human brain isn’t designed to be claimed. The very action shuts it down as efficiently as an axe through the skull.