Don't panic, he hasn't died, it's another effect of the collar that's kicked in, rendering him a ghost with... well, not much exactly in terms of ghostly abilities, he's basically just the same as usual, but seethrough and not really needing to breathe, drink or eat anymore.
...which is slightly concerning for its own reasons, but hey, at least he's a cute ghost, right?
Fortunately, though he might not know it quite yet, he's not going to be forgotten, instead with his ghostly state's new benefits he might just find himself getting quite thoroughly put to work, to say the least, and make an interesting curiosity too~!
I'm sure he'll make a lovely servant for his captor~
Made by the wonderful NX-42 Inspired by this lovely idea here