Complex sentence: Bergensten was originally hired as Mojang's backend developer for "Scrolls", but began programming more and more significant parts of "Minecraft" until he took over its development completely on December 1,2011, after Markus Persson stepped down from this position.
Split into simpler sentences: Bergensten was originally hired as Mojang's backend developer for "Scrolls". Finally he took over its development on December 1,2011, after Markus Persson stepped down from this position.