The Fallen Archangel (A HSDXD Alternate Story)

Even though they possessed a significant age gap at the time and were of different races, Avalon was once Rias Gremory's close childhood friend, even being considered to be like an older brother to an extent. He was also one of the most powerful magic users known to the underworld's history. Due to possessing power beyond what anyone had ever seen, a decision was made by the devil king at that time to seal Avalon using Rias and others that he cared for as bait to get him to surrender, Due to not wanting to endanger Rias or the ones he cared for, Avalon obeyed as he was told and did not retaliate during the sealing process.
Despite that the seal itself could be broken at any time by Avalon, he refused to do so because if he ever did, it was stated that the ones he cared for would be hurt or even killed if he was ever spotted unsealed. Avalon was sealed within a magical pendant, which was given to Rias as a way to remember her friend, over the first few years Rias held onto the pendant but as time went on, year after year, decade after decade. century after century, she grew distant of it and eventually forgot about it, with it now being lost somewhere she once resided in centuries ago.
While it is true that Avalon is sealed and the pendant may not be with Rias anymore, he is able to glimpse of what is occurring around her from time to time, ensuring that she is safe and happy.