…pero es que a Tiberio se le había hundido un anfiteatro con centenares de muertos.
I guess this sentence says that Tiberius drowned an amphitheatre with dead bodies (figuratively). However, I don’t understand the use of either of the pronouns ‘se’ and ‘le’, or why it uses the phrase ‘a Tiberio’. Usually if you turn a verb into a construction like ‘gustar’ besides putting emphasis on the person after the ‘a’ you are also saying the they are also not the subject of the verb but instead are affected by the action of the verb. In which case Tiberius was drowned by the amphitheatre with hundreds of bodies. But that doesn’t explain the ‘se’ as ‘hundirse’ means to sink yourself.
Why can’t it just be ‘Tiberio había hundido un anfiteatro con centenares de muertos’?