Knight-Commander Martel is first encountered during the meeting with Divine Beatrix III at the Grand Cathedral in Val Royeaux, where he accuses the Seekers of being reckless by not asking for Templar reinforcements in their attack on the blood mage ritual. High Seeker Aldren argued that the Seekers of Truth maintained stealth and discretion in comparison to the Templars, a suggestion at which Martel took offense. Grand Cleric Callista warned them against openly arguing in the presence of the Divine.
Later at Callista's request, he is dispatched to lead the hunt to apprehend the renegade Seeker Cassandra Pentaghast and Regalyan D'Marcall. While hunting the pair, Martel accidentally reveals that he is Frenic's co-conspirator in the Chantry ranks. The duo were able to steal the elven communication stone he possessed to communicate with Frenic and trapped him in a cave. The Knight-Commander survived and he anticipated that they would attempt to alert the High Seeker. Infiltrating the Seeker stronghold, Martel murdered Aldren before he could alert the Divine of his treachery, framing Cassandra and Regalyan for the deed.
Martel later met with Callista in a Chantry confession booth to report on his success. Together, they plotted to seize control of the Chantry, with Callista as the new Divine, and planned on betraying Frenic.
Martel later visited Cassandra and Regalyan in the dungeon, revealing the conspiracy's grand plan: Frenic would use the elven mage, Avexis, to command high dragons to assassinate the Divine and other Grand Clerics at the Ten year gathering. And from the ashes, Martel would lead the Chantry into a new age where the Templars rule supreme. Cassandra stated that the Seekers would never allow him to seize power but Regalyan suspected that one among the Grand Clerics was involved in the conspiracy would become Divine if all others in line for succession were wiped out. Cassandra demanded the identity of Martel's co-conspirator but he refused to answer.
Martel presides over Cassandra's execution. However, before she is beheaded, the Circle mages summoned by Regalyan attack, enabling Cassandra to escape. Though his Templars were incapacitated, Martel barred their escape and engaged Cassandra in a duel. The Seeker accused him of betrayal, to which he argued he was working to prevent events like Kirkwall from ever occurring again. Cassandra managed to sever his sword arm and ordered the Circle mages to bind him to stand trial. However, Martel broke free and attempted another attack, but Cassandra swiftly decapitates him.
(Source: Dragon Age Wiki)