Is it looking a bit like this:
note the junction of diseased and healthy wood where the stem has been girdled. Branches that have been invaded and girdled by the fungus wilt and die back. The diseased wood at the base of the branch near the soil line turns brown.
Here's an image that shows the junction of diseased and healthy wood:
If so, it is probably phytophtora, a root rot fungal disease.
Planting too deep, especially if your soil is heavy, clay-ish, can contribute to the development of root rot:
"Planting too deep can stunt tree growth, lead to soil compaction and a lack of oxygen for feeder roots, and even adversely impact tree stability. Under conditions such as these, if soils become saturated because of excess rain, trees may die from waterlogging or may be predisposed to infection by phytophthora root rot." https://www.clemson.edu/extension/peach/ ... _deep.html
are you in an area like me where you have had a very wet summer? Lilacs are pretty vulnerable to root rot in those conditions.
You can try using a fungicide as a root drench.
You might try pulling some soil away from the base of the trunk where it is buried too deep. But don't let it stay around the tree where it would hold water. Create some slope away from the tree to keep water from pooling. You can try amending your soil with bark chips to improve drainage.
When the leaves fall, be sure to rake them up and destroy (not compost).
Best wishes and welcome to the Forum!