During the reigns of Viserys I and Aegon II Targaryen, Lord Caswell was the Lord of Bitterbridge and the head of House Caswell. In the TV version of House of the Dragon, he is played by Paul Hickey and is called Allun Caswell.
Lord Allun Caswell in the Dance of the Dragons
People who supported Aegon II Targaryen put Lord Caswell in the Red Keep’s cells at the start of the Dance of the Dragons because he supported his half-sister, Princess Rhaenyra Targaryen. There was a time when Lord Caswell had to decide whether to swear loyalty to Aegon II or be killed by the King’s Justice. He chose to keep his word and was killed for it.
Their heads were stuck on spikes above the gates to the city. Tom Flowers, the Bastard of Bitterbridge, led the Caswell men in the field because Lord Caswell’s son and heir was still a child. Lady Caswell, Lord Caswell’s widow, ran Bitterbridge in the name of her young son.