Shade Garden under Sassafras Tree

I would like to plant a shade garden under a large sassafras tree in my back yard. The tree throws so much shade that nothing will grow under it, not not even grass.

My main question is...Will I hurt the tree if I put a garden around the base? I will need to put additional soil and mulch around the tree, and I don't want to cause damage to the roots.

Another question...Should I till around the tree in addition to adding more soil? Again, I don't want to cause damage to the tree roots.

Has anyone done this before, and do you have any recommendations? I would also like to do the same under a large oak tree.