If you want to eliminate the sudden drop-off, peel back the sod perhaps 6 feet all around and regrade the soil to a gentle slope down toward the level of the tree, then re-lay the sod.

Building a ring wall, putting in a circle of rocks, etc., is rarely good design.

Alternate idea -- eliminate the sod around the tree altogether. Instead, extend the planting bed (on the left in the photo) in a natural flowing line to incorporate the tree into the bed. Looking at the photo, you could more or less follow the shadow of the tree to extend the bed.

Seeing only the base of the tree in isolation doesn't give me much to go on in terms of design.

Enlarge the area and put in a tree seat-

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I would second Catkim's suggestion to just gently reintegrate the tree into the surrounding yard - i.e. just by a simple regrading or by incorporating it into the planting bed.

- Audric

From personal experience, incorporating a maple into a planting bed is grounds for disaster. Nothing is going to grow in its canopy. I am liking the idea of the tree seat for my own situation. Someone incorported a maple in our planting bed and nothing does well under it. You will be lucky to get thugs to fill in. Tree roots of maples surface and take every bit of soil available. The base of that tree doesn't look like a maple to me. If it's buried that deeply, tree roots will still come to the surface and Maples don't do well when roots are buried.