now this explanation is for the people who haven't played any elder scrolls games or read the novels well
way back in the first era ayleids who are a type of elf mer is also another name for a elf well they were making humans into slaves obviously humans eventually had enough of this and a rebellion led by saint allessia had made most of the elves extinct
not all elves mind you there were some that survived but they died off eventually they are just one of the many extinct elven races basically Solomon is joining the rebel faction.