As Miquella is associated with unalloyed gold and known to be supernaturally charming, it stands to reason that he had something to do with preparing Torrent for summoning.

Source: https://gamerant.com/elden-ring-torrent-lore-backstory-melina-miquella-ranni-connections/

Miquella despised the Golden Order and opposed the Greater Will to find cures for his and Malenia’s afflictions.

Source: https://screenrant.com/elden-ring-dlc-story-new-ending-miquella/

The only details regarding Shadow of the Erdtree are its name and concept art which shows a young figure atop Torrent, which could possibly be Miquella.

Source: https://www.cbr.com/things-that-should-happen-in-elden-ring-shadow-of-the-erdtree/

There is solid evidence to support the belief that Elden Ring's comatose demigod Miquella is the true identity of the mysterious St. Trina.

Source: https://gamerant.com/elden-ring-miquella-st-trina-same-person-why-similarities-clues/