Canon Background
https://awoiaf.westeros.org/index.php/Marwyn

In-Campaign
Before the grand conclave of 284 AC, which co-occurred with the royal wedding of Robert and Cersei, Marwyn encountered Thaddeus in Red Keep’s library. When he noted that Thad was reading his own work, The Book of Lost Books, Marwyn probed the younger maester as to his opinions on the subject matter. Suitably impressed by Thad’s responses Marwyn urged Thad to b present at the gran conclave.

At the grand conclave Marwyn argued for the funding of an expedition to reestablish contact with the Children of the Forest. To learn their magic and gain their wisdom. He called on Thaddeus to testify that Marwyn is not a lone malcontent, but rather that many in the citadel would have interest in such an endeavor. Ultimately Marwyn and Thaddeus were overruled and jeered at; Grand Maester Pycelle led the counterpoint and Seneschal Benedict ruled in favor of Pycelle and the conventional wisdom that the children are extinct. Afterward Marwyn extended to Thaddeus an offer of sabbatical and further study under his tutelage which Thad accepted.

After a year of study under his guidance Marwyn bade farewell to Thaddeus in October of 285 AC. In his farewell Marwyn alluded to Thad’s handling of himself during some conflict with a “necromancer” as having earned his trust. He lent Thad “The Elder Children” by Arch Maester Kenneth from his private library as a parting gift.