It sounds like you want to grow something besides turf on this mound under your oak trees so that you won't have to keep it mowed. You could plant the mound with shade-loving ground covers like Aegopodium podagraria 'Variegatum', wild ginger (Asarum europaeum), sweet woodruff (Galium odoratum), lily of the valley (Convallaria majalis), and Lamium. Adding a few drifts of shrubs and larger perennials would give the landscape form. You could plant long serpentine clusters of Hydrangea, Hosta, foam flower (Tiarella cordifolia), Epimedium, bleeding heart (Dicentra species), hardy geranium (Geranium species), and ferns. Your 50-by-20-foot mound will make a fairly large shade garden. Remember that the installation and initial maintenance (watering and weeding) over the first year or two may be labor intensive, but after everything is established the maintenance chores will diminish.