It’s probably not Steel, although Brandon has confirmed it’s not Lerasium. It is definitely connecting Kell’s soul to his body though:
Questioner
In Bands of Mourning, we learned that the Sovereign, who they confused as being the Lord Ruler, came after the Catacendre. [He] was their god, was their king and god. And then Kelsier looking for a string. Is the spike somehow connecting Kelsier's soul to Spook's body.
Brandon Sanderson
No, good question. It is connecting his soul with his body, his current body, but it is not Spook's body. That's a great theory.