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

In-Campaign
In the days before the Taking of Gulltown an envoy from House Jasper arrived at Heart’s Home. Lyonel was preparing to march when a bard met him on the blacony as he was contemplating the impending battle. He introduced himself as Dolins and said he hailed from House Jasper. Gifted of gab, Dolins was able to sway Lyonel from royalist to rebel in the span of a single conversation.

At the Battle of the Trident Corbray forces were held in reserve. When the Dornish envelopment began to push them back Lyonel’s lord father was cut down. Lyonel rushed to his aid and hauled him to safety. His father laid dying for a week after; during that time Lyonel was bequeathed his father’s lands and titles. Much to his his chagrin Lyonel’s brother Lyn was given their ancestral sword.

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.

Lord Lyonel was come upon by Ser Dolins and Maester Thad in the armory at Snakewood at the wedding of Teddy Jasper and Jana Lynderly. The lord was searching for his Uncle’s bow (a fine thing, engraved with a snake wrapped from tip to tip), with no luck.