Small white worms (maggots) emerged from my monarch pupa, what are they?

These are tachinid fly larvae. Soon after they emerge from the monarch caterpillar or pupa, they will turn to dark brown pupae. An adult tachinid fly lays her eggs on a monarch caterpillar, then the tachinid larvae burrow inside the monarch caterpillar and develop within it. They emerge from late stage monarch caterpillars or early pupae, killing the monarch.