Maddox is a Tranquil.
Prior to becoming tranquil, Maddox was a Mage at the Kirkwall Circle of Magi. Maddox fell in love, but due to Knight-Commander Meredith Stannard's harsh policies he was unable to communicate. A templar named Samson, sympathetic to his situation agreed to pass letters between Maddox and his sweetheart. Samson was eventually caught and dismissed from the Templar Order while Maddox was made Tranquil.
If the Inquisitor sides with the mages, Samson becomes The Elder One's right hand. Maddox is sent to the Shrine of Dumat to further his research, and using his knowledge of Red Lyrium builds Samson a nearly indestructible set of armour.
Cullen recommends that the Inquisitor try and find a weakness in Samson's defenses. Eventually, the Inquisitor's investigation leads them and Cullen to the Shrine of Dumat. To avoid the Inquisition discovering his secrets, Samson orders both the Shrine and everything inside to be destroyed. Samson escapes while Maddox volunteers to remain behind with several Red Templars and destroy any information or material which could be used turned against them.
After arriving at the site, The Inquisitor and Cullen fight their way through the burning structure and eventually encounter Maddox in his laboratory. Maddox reveals he anticipated the Inquisition's arrival and ingested a fatal amount of poison to prevent himself being used against Samson. Maddox refuses to answer any questions and passes away shortly after.After the Inquisitor returns to Skyhold, they can order a funeral for Maddox.