King Jaehaerys I is only on the House of the Dragon for a small flashback. I knew who he was before the show was ever made, so I was delighted to see him at in live-action. A grandson of Aegon the Conqueror, he was actually the longest living King of Westeros. Very much like Louis XIV of France, he outlived his children. Which made his succession more tricky.

As his eldest descendant was a woman, but he had grandsons. So he summoned a great Council for Westeros so they could pick the next king and avoid civil war.

For which his grandson Viserys is named the next king, unsurprisingly. Which ironically just delayed a civil war, instead of avoiding it entirely.