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

In-Campaign
Whilst preparing to march on Gulltown with his family a young hand maiden entered Lucas’s study. She plied kind words and flattery, no doubt in an effort to win him to her cause. He did begin to grow to like her, ever so slightly, but it was clear to him there was an opportunity. He gained the upper hand in their discussion of a possible alliance by manipulating her into making threats. Clearly embarrassed by her outburst a pact was made with the woman. Lucas will speak in favor of neutrality in exchange for 400 gold dragons from the House Jasper coffers.

A contingent from House Jasper called upon Heart’s Home in February of 286 AC. Lord Lyonel and his brother Lucas treated with them over the course of two days. Both houses had recent run-ins with House Lynderly (though in truth House Corbray was the victim of a false flag attack orchestrated by Jasper men) and House Jasper sought the support of the Corbrays in taking them down. Ultimately the following terms were agreed upon. One, the men who attacked Ser Lyn would be taken captive so that he could face them himself after he recovered from his wounds. Two, Corbray soldiers would be a reserve force in the siege on Snakewood as Arturo Stone was held there and Lyonel was concerned his uncle may be executed if their presence were known.