FullMetalAlchemist-Passage Fee

The Passage Fee

Ed and Al wanted to revive their dead mother. In order to do this, they have to re-create her through alchemy using human transmutation, which is forbidden.

In utter ignorance and innocence, they pushed through with the transmutation, and in the end, they realized only too late why it was forbidden to transmutate humans. Al's body and Ed's leg was taken in exchange for a failed and horrible transmutation, of a being which cannot be called human at all.

I haven't watched the first episodes of the series, so I'm basing this one from the manga.^^