Taddevan appears as one of the companions of Emilia in to Slay an Immortal, as the group looks for the Sword of the Gods. He's a sixth-level mage with expert Order Magic, advanced enchantment and wizardry, and basic charm. He begins with a Mage's Staff and a Potion of Healing.
He spent much of his career huddled over books, hence his pale skin and bad vision. After the Reckoning, he was forced to use his magical skills more often and discovered that he was quite talented. But he still dreams of a day when he can return to the libraries and retire in peace.
Emilia asked Taddevan to join them on their quest because the enchanter was a learned man, to provide their party with more spellcasting powers.
Every night, when they made camp, Taddevan and Emilia placed protective spells over the camp to make it less likely that they were discovered during the night. As a learned man, Taddevan was not superstitious, and rolled his eyes at Fridilok's little rituals.
After they located the sword, the companions, apart from Kodge and Tharj, became part of a skirmish unit who harassed Gavin Magnus and his troops. Magnus found them, and all were captured. It's unknown if they were executed, or were freed when the Crystal Pendulum was destroyed.