Camelot
Adventurer
If you kill the creature, I would rule that it is no longer a creature, and you can't use it to teleport. Flavorwise, you travel through shadows but only those of creatures because they contain the creature's soul. If the creature's soul departs it, it's shadow becomes useless to you.
However, if you were to knock it unconscious, I would allow it. There is a downside, however. When a creature is killed, I remove its mini from the board and we ignore its dead body. If it is unconscious, its dead body remains on the board prone. You can enter the square of a prone creature (at least I'm pretty sure; in "Stand Up," it says there's the possibility of your square being occupied by another creature while you're prone), but it is difficult terrain. Therefore, you can teleport using it, but you have created an unnecessary square of difficult terrain and I let monsters make death saving throws if they're unconscious, so it could potentially come back in the middle of the fight by spending its one healing surge (or two or three at higher levels).
This is how I would run it, but it can be done many ways.
However, if you were to knock it unconscious, I would allow it. There is a downside, however. When a creature is killed, I remove its mini from the board and we ignore its dead body. If it is unconscious, its dead body remains on the board prone. You can enter the square of a prone creature (at least I'm pretty sure; in "Stand Up," it says there's the possibility of your square being occupied by another creature while you're prone), but it is difficult terrain. Therefore, you can teleport using it, but you have created an unnecessary square of difficult terrain and I let monsters make death saving throws if they're unconscious, so it could potentially come back in the middle of the fight by spending its one healing surge (or two or three at higher levels).
This is how I would run it, but it can be done many ways.