What is the sequel series name that Rick Riordan going to write?

Rick Riordan is currently writing a sequel series called "The Trials of Apollo". It is a continuation of the "Percy Jackson & The Olympians" series, focusing on the god Apollo who is turned into a mortal teenager.

However, it's important to note that Rick Riordan has also announced several other upcoming series in different universes:

* "The Kane Chronicles" sequel series, "The Demigod Diaries," featuring the Kane siblings and their adventures with Egyptian gods.

* "The Trials of Apollo" sequel series, "The Tower of Nero," featuring the adventures of Apollo and his friends.

* "Magnus Chase and the Gods of Asgard" sequel series, "The Ship of the Dead," featuring the adventures of Magnus Chase and his friends.

It's best to check Rick Riordan's official website or social media for the latest updates on his upcoming works.