In the following text, the dative relative pronoun (or whatever it could be) does not start the clause:
Hic templum Iunoni ingens Sidonia Dido
condebat, donis opulentum et numine divae
aerea cui gradibus surgebant limina nexaeque
aere trabes, foribus cardo stridebat aenis.
This passage tells us about the foundation of a Temple dedicated to Juno in Carthage, describing the richness of the place (donis opulentum). (Aeneid, book I, lines 446-449)
Firstly, the pronoun does not show any antecedent (elliptical or not). Furthermore, it does not begin its clause.
Is Cui actually a relative here? If it isn't, what could it be?
I considered that cui can have only singular nouns as antecedents. But all the nouns reasonably close to cui are plural: I tried to take numine and divae as the antecedent, but I couldn't figure out what the clause would mean.