I've had this idea a long time ago, not in a galaxy far far away. And would like to know if it has been done before, note that this is very undeveloped at the moment.
SCP-XXXX is contained in a osmium cell surrounded by an electromagnetic field.
I am going to ask: Why osmium? It is a rare element on Earth, so it will be difficult to make a cell out of it. Is there a specific need for osmium that steel and concrete cannot handle? You must take note that SCPs are not about using exotic stuff to contain them, but to use reasonable things for containment.
Even if it is less likely to pass through denser material, the sheer rarity of osmium makes an osmium cell hard to believe. Perhaps if you offer a truly engaging story to this SCP, one might opt to forgive that aspect.
And upon reading it, I find the article to be stale. You are quite barebones with it, only depicting this humanoid figure that kills people via a certain method. You are not elaborating into the person behind this SCP. Like, is it always static? Was it a human turned static or a static entity that held a human form? Does it want to kill people? Does it mean to kill people? How does the "phasing through objects" work as a process?