I had a different idea, but now that you mention it. "Preparations" might be something to keep in mind. Doesn't really need to be Knowledge related, but maybe it is worth the effort to consider the option of allowing the players to spend some time preparing specifically for the Trial. Be it potions or research. To lower the difficulty a bit. Hmmm...
To be fair that's the thing I definitely would want to avoid. "Having to figure out the event". Not just because that's unfair. But also because of how volatile it makes the whole thing. Just imagine what would that lake even be if one of the players didn't prepare a "breathing underwater magic"?! What if no one did??!! What if none of them opened the egg and was like "what are we supposed to do here in front of this lake?"
That would be a very awkward day.