...a Tiberio se le había hundido un anfiteatro con centenares de muertos

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1

…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’?

2

It is “a Tiberio” because he is an object of the verb, NOT the subject of the reflexive hundirse. Hundrise is reflexive here because the ampitheatre is both subject and object. = se le habia hundido because “it had collapsed.” Just like se me olvido “it forgot itself.” And just like you would say “se me olivido el coche” or “The car forgot itself to me” you would say “se le habia hundido el anfiteatro A TIBERIO” because the ampitheatre collapse itself on (on the watch of) Tiberius. The “a Tiberio” is further needed to claify because the ampithere is a third person singular and uses “Le” just like “he/him/it” Now, both