Lots to choose from , the main thing is it dry shade or damp as this will be the decider whats best to grow.

Gg

Underneath a lilac, nothing. They are massive and will take all the light and moisture from the location.

Is it a young lilac, and therefore still small? Is it on a single trunk or has it lots of suckers? If so there are things you could plant until the tree gets too big. Need to know if dry or damp as GG says.

thank you all for your answers the ground is dry but have white lilies growing self seeded and two astermeria
little princess the lilac tree is four or five years old last year we only had one bunch of bloom this year it is covered in bloom

If you were wanting to make the rockery within the raised bed its hardly worth it - most rockery plants like full sun and the plot will become increasingly shaded and filled by the lilac.
Had you thought of perennial geraniums? They won't mind the dry ground and most won't mind some shade and there's a good range of colours in the red/blue range. Just be careful not to get the taller ones as they would hog the space when they fall over in the summer.
Saxifragia urbium (London Pride) would slowly make good neat ground cover in dry shade.
Empty space this summer could be filled withbegonia semperflorens.