I didn't want to spoil the ending of Tigrane for anyone before it went up, but now I can explain it. It went something like this:

1. I'm about to execute Tigrane for trying to kill Queen Tomiri (who incidentally is in love with him).
2. Meroe suddenly shows up, admitting that it was actually she who had tried to kill the queen to avenge the brutal death of her father King Cyrus; Tigrane had tried to stop her and ended up looking guilty in the process.
3. Seemingly from nowhere, the head guard brings in a letter confirming this. Tigrane is released; Meroe is captured.
4. Again out of nowhere, the Egyptian king Policare appoears, saying "Great news, Tomiri! Turns out Tigrane is your son!" Apparently he got this information from a dying pirate (!?) who had captured Tigrane as a baby. Proof is offered in the form of a previously-unnoticed royal mark on Tigrane's arm.
5. Tigrane confesses his love for Meroe, and Tomiri is so overjoyed that she orders Meroe released, giving up her rather Oedipal claim on her son.
6. For no apparent reason but this, Meroe decides her father doesn't need avenging any more.

And this is supposed to be a serious opera??

It reminds me of these sliding number puzzles:

Some people like to try to do these in the fewest moves possible. Those people might have been able to end Tigrane with only a letter and one unbelievable character transformation. Scarlatti, clearly, was unconcerned about the number of moves he took...