King Bela IV is a hero unit in Age of Empires II: Definitive Edition. He is available only in the Scenario Editor.
The unit is exactly identical to that of a standard King: he cannot attack or perform any function except simply move around and garrison inside buildings. He can even move at the same quick speed as standard Kings.
While King Bela plays a role in the latter half of the Kotyan Khan campaign, he doesn't appear himself in any scenario.