Weylan Darrow (often simply called by his surname) is a lord of Terrasen who served as a sort of steward for the country during Adarlan's reign.
Empire of Storms
When Aelin returns to Terrasen in Empire of Storms, her and her court meet with Lord Darrow, Murtaugh and Ren to discuss her return to Orynth and plans for the war with Erawan. However, Darrow consistently insults Aelin and claims she is unfit for the throne. It is clear to Aelin and the others that Darrow himself is interested in the throne, and because of a document that he and the other lords of Terrasen signed (excluding Murtaugh), Aelin is unable to claim her rightful place as Queen.
After the meeting, Darrow and Aelin part ways - Aelin heads to Ilium and Darrow returns to Terrasen. Darrow does not believe magic can play a role in the upcoming war, since it did not make a difference when Adarlan invaded, and claims that men can win wars without it, something he and Aelin clearly disagree on. Before Aelin leaves, Darrow bans her from setting foot and Orynth and tells her if she tries to take back her throne without the lords' invitation, it will be seen as an act of war.
After Aelin goes to Ilium, he is not seen again.