it will be called wall of steel if  kill it it will unlock extrememode and drop killhammer and 3 new ores and 1 new biome the three ores will be called plastic, watching ore and finally bloody ore and can make all the stuff the new biome will be called bloody biome that you can find the bloddy ore and watching ore in it wall of steel will look like wall of flesh exept light grey it has the steel hungry instead of the hungry  and they are the same color as the wall of flesh it has steel leeches too same color as the wall of steel too. it looks like that (under)