Ever since Murtagh was captured by the king and put under house arrest, he had been conniving plans on how he would create complete mayhem in the kingdom. He had rebelled against the kingdom and had great influence amongst the border tribes. However, Cora came up with the idea of giving her granddaughter to him in marriage, and that was what stopped his activities.
However, the girl was lost. Murtagh wanted to go back to being a rebel, but he didn't because as Iona's would-be betrothed, he still had excellent benefits. He was respected and feared by many in the kingdom. Moreover, his business flourished. He would make deals with traders to give them extra benefits in the kingdom and accept bribes. Also, without Iona, it wasn't like he had a shortage of women. He had more than enough women for his needs.