Complex sentence: It is not known for what purpose the "Prélude d'Eginhard" was composed; the title suggests it was intended as a piece of incidental music for a play, but no such relevant stage drama has come to light.
Split into simpler sentences: It is not known for what purpose the "Prélude d'Eginhard" was written. The title suggests it was intended as a piece of incidental music for a play, but no evidence of this has come to light.