We've noticed this spreading since late summer; we are new to the home so have no idea what this is. I'm having trouble finding what it is online. It's darkened to purpleish green since late fall but I remember it being a normal dark green.
It looks like Ajuga retans (common name Bugle).
I grow as a very effective ground cover, it has pretty blue flowers in Spring. If it starts encroaching I just pull it out.
