To be honest, I no longer understand the purpose of “tethering” now that we know that the Season 4 explanation — tethering the Darkness to a person in order to contain it, then forging/creating the dagger to control that person — is no longer valid.
Here’s a theory, in light of recent spoilers. We know that Hook’s name is on the sword. If the sword now works much like the dagger, then the obvious explanation is if Hook killed Merlin. The current portrayal of Hook in Camelot is of someone progressively spinning out of control (another Desperate Soul?), and he’s already lashing out at Merlin anyway. Now that Merlin’s under Arthur’s thumb, it seems like neutralizing him would be a good idea. Many here have speculated that Merlin’s dead by the time they get to SB. So perhaps Hook decides to kill Merlin on his own initiative, or Merlin essentially requests Hook to kill him (to prevent the damage that Arthur might do with Merlin at his beck and call), as the most likely candidate to take one for the team, since Merlin’s and Hook’s predicaments are similar.
The only problem with this is that it’s maybe a little too obvious …