King Hekaton might fit your requirements

Unless you need a character from one of the playable races.

King Hekaton, from Storm King's Thunder pg. 222, is the king of the storm giants, and therefore on top of the ordering. Since you are attacking a flying fortress of cloud giants, the cloud giants might have angered him in some way. I am not familiar with the cloud giants agenda in Hoard of the Dragon Queen.

For as long as Hekaton had reigned, fear of the king’s wrath and respect for the ordning was enough to keep lesser giants from rising up against him.

King Hekaton also have access to a lightning attack and control over weather. And storm giants rely on Rocs to fly, which would help him access the flying fortress.

He should also be alive around 1489 DR.

King Hekaton might fit your requirements

Unless you need a character from one of the playable races.

King Hekaton, from Storm King's Thunder pg. 222, is the king of the storm giants, and therefore on top of the ordning. Since you are attacking a flying fortress of cloud giants, the cloud giants might have angered him in some way. I am not familiar with the cloud giants agenda in Hoard of the Dragon Queen.

For as long as Hekaton had reigned, fear of the king’s wrath and respect for the ordning was enough to keep lesser giants from rising up against him.

King Hekaton also have access to a lightning attack and control over weather. And storm giants rely on Rocs to fly, which would help him access the flying fortress.

He should also be alive around 1489 DR.