Underking

From the Johore capital on the island of Pulau Penyengat in the Riau Archipelago, where Parani's brother became "underking" to a puppet Malay sultan, the Bugis extended their control over the tin negeri of Kedah and Perak, [...]

Bede himself used it in this context, among others, referring for example to Peada, Penda's son and the underking of the Middle Saxons, as a princeps.

“I am an advisor in his role as an InterSec director, Lord Guardian, not his counselor as underKing to the Inverni.” Aran made a dismissive gesture with his hand. “A distinction without a difference. [...]”