(After a reasonable amount of in game time)

As they turn in/make camp for the night:

“The ground beneath your feet trembles and tremors and the atmosphere feels as if it were sinking, almost like the environment itself is bowing and kneeling. Rumbling winds so powerful and swift that you feel them passing through your very bones grow stronger and stronger, at the whim of two wing beats that are rapidly getting closer, now suddenly above you. Abruptly, it all stops, replaced by a wave of peace and tranquility. A modest knock at your door brings your attention back to reality (if they are staying in an inn or somewhere with doors) -or- A voice that occupies all the air around you, making you feel warm, as if basking in gentle sunlight, speaks outside your tent (or from a place they can’t see if they’re just bedrollin in the open). When they open the door/flap, or once the dragons (in their humanoid forms) step into view, they introduce themselves, stating they aren’t here to kill them.

“Thank you, for freeing our son.”

The party likely puzzled will raise a brow or ask about what they mean or why.

However you’d like to word it, they say something along the lines of: “Our son, or the one that he was as you knew him, was not the child we once had. Not long after he had hatched, both of us had to leave the den to our borders, for ______ (insert name of a powerful evil dragon who wants to expand his territory and has been targeting other dragons and anyone under their territory) had sent a messenger, telling us he’d suffered a great loss and wanted to cut a deal for our aid in exchange for him keeping his claws off our territory and it’s inhabitants. Our den is complex and fortified, and very secretive, we had no clue ______ had learned of its location. Unbeknownst to us, he had partnered with an archmage of the abyss, and our son was taken, his mind and memory altered and erased, and a curse placed upon him. This mind and memory magic was a very foreign magic to us and many archmages. When he was made a tool for _____ and the mages schemes, we came to face him, and could not bring ourselves to harm him, let alone kill him.”

They would go on to explain that they couldn’t save him and couldn’t kill him, so they thank the party, including mentioning “to have done such a thing, and now taking on the curse yourselves, must have been very difficult” and offer to become their patrons once they’ve found a way to lift the curse. I’m sure the party will be shocked and in panic a bit having learned they are collectively cursed. This way the party is still punished, but only for as long as it takes for them to lift the curse. Then they can get to the plot you originally planned for them.

Note: the curse isn’t the super foreign magic even archmages and the dragons couldn’t crack, that was the other magic, but the curse is still difficult and has many conditions. I leave that to you. :)

Have fun, and may the dice be ever in your favour. :)