Rex, postquam boves geryonis accepit, laborem undecimum Herculi imposuit, graviorem quam quos supra narravimus. There is a footnote after quam which says to substitute ‘ei erant’. I believe this is a situation where a form of qui is used instead of a form of is which the book I am using fully explains. I think erant is the antecedant to quos and that graviorem modifies laborem but I have trouble after that. The best translation I can come up with: The king, after he received Geryon’s cattle, imposed on Hercules an eleventh task, rather arduous there was to him which we spoke about above.
It seems to me that the footnote is only there to increase confusion.
graviorem quam quos supra narravimus = graviorem quam ei labores, quos supra narrivimus
The note surely doesn’t say to substitute ei erant, but to mentally supply it for the sake of the grammar. The ei is nom.pl., sc. labores; ei is the “understood” antecedent, not erant (verbs can’t be antecedents). “harder than (were those [tasks]) which we’ve recounted above.” It’s a bit pedantic, and maybe more confusing than helpful, but it is grammatically correct.
(At the risk of compounding confusion, English is more likely to elide the relative pronoun than its antecedent: “… an 11th labor, a tougher one than those [which] we told of above.”)
I goofed by forgetting that quam means something else than a relative pronoun.