It takes place a little before episode 1 and shows the first battle between Reinhard and Yang.
My Conquest was the first Lotgh thing ever animated and is like a pilot episode for the original OVA, but it's still canon and is referenced occasionally. Legend of the Galactic Heroes Gaiden (two seasons called 'Spiral Labyrinth' and 'A Hundred Billion Stars, A Hundred Billion Lights').
