It's hard to see enough clear symptoms from the images to be certain, but a likely cause is either Volutella Blight (a closely-related pathogen to the one causing Volutella Blight on boxwood) or root rot due to soil that isn't draining well or which is staying too damp. Volutella is probably more likely since root rot would probably have decimated the patch more rapidly than what you describe; plus, unless you've been watering a lot, the weather of the past two years has trended notably dry.

Information about Volutella management is included in the page linked above. Essentially, it involves stripping off the existing above-ground growth (mowing it down to the ground, if possible), cleaning up all debris, and then letting the plants regrow at their own pace. While some fungicide options might work decently well (as long as they are labeled for Volutella in residential landscape settings), they are preventative measures only, so cannot cure existing infections. Preventative sprays that suppress new infections are not usually recommended, both because they are a hassle to apply (several applications are usually needed over the course of spring or into early summer), might not always work (if rain interrupts the spray schedule, allowing infection to take place), and because they expose other organisms to potential harm. (For examples, some fungicide ingredients can harm pollinators like bees.) Japanese Pachysandra is invasive in Maryland, and rather than treating it, we encourage gardeners having trouble with established plantings to replant those areas with alternative groundcovers.

Miri