Player trivialized my boss battle

Disclaimer: I am not complaining -- I celebrate player victories! I just want to share this iconic moment for one of my players.

This past weekend, I ran the concluding session of a three-shot adventure I've already run before. The adventure itself is an Isekai-type adventure in which the players are transported to a fantasy world I've homebrewed. Over the first two sessions, players have blank character sheets and instead have to "discover" their abilities and skills. It's a lot of fun and we had a great time!

By the third session, I gave my players their full character sheets, which included any spells they hadn't already discovered through gameplay. The combats up to this point were designed to be relatively easy, since the PCs weren't fully fledged versions of themselves. For the ultimate boss battle, I intended to make it a fair but difficult challenge, since they were able to play with their full sheet (at level 5). The boss I designed was a modified Horned Devil with a magical shortsword that created 15-ft circles of magical darkness instead of a fork. He was meant to set up for a few turns creating multiple pockets of darkness by swinging his sword, teleporting between each pocket, and then attacking the players with advantage. He was also able to summon a few Shadows each round with a legendary action. The strategy of the players was originally to force the Devil to come out of hiding and attack him in the light.

That is until my Druid showed me the Daylight spell. I had given this spell to the Druid without fully reading it, thinking it would be handy for the adventure but not overwhelmingly powerful. Little did I know that this spell does NOT require concentration, and it affects an area of 60ft -- just about the entire area of the throne room they were fighting in. This essentially trivialized the entire function of the Devil's magical shortsword, which the party had spent the entire adventure trying to prevent the Devil from acquiring.

Of course, as the DM, I could've ruled that this magical darkness was too powerful for Daylight to dispel, but honestly I really had to hand it to the player! It was still a fun battle with the Devil's standard abilities, but the players were much more confident after the Druid cast that spell. Have your players ever taken advantage of a boss' weakness that you totally overlooked?