Thomas Aldertree, born Thomas Daltrey, is a Shadowhunter—one of the first few to Ascend again after decades. He was among the first batch of students at the reopened Shadowhunter Academy and trained alongside other Shadowhunters and mundanes, including Simon Lewis.
In 2008, Thomas was recruited into the Shadowhunter Academy for potential Ascension. After two years of training, he and nine other mundanes were set to finally go through Ascension—a ritual with known risks. While he decided to go through with it, three of their classmates had backed out at the last minute and left the night before. The students were called alphabetically, and Thomas came second. After some ceremonial rites and swearing to uphold the oath dictated by the Consul, Thomas drank from the Mortal Cup and survived, successfully Ascending into a full-fledged Nephilim.