I just completed the FMA:B anime (been meaning to do this) and I'm still confused as to what they actually created. Originally, they thought it was their mother but then Edward realized that it actually wasn't their mother that they transmuted.

We then find out that Alphonse was able to see Edward from that thing's perspective. Was that thing a failed homunculus they created? And why was Alphonse able to see everything from its perspective?