My Party took My dangerous encounter and ended it in 2 rounds.

So I am a DM on a Discord server that is run like an MMORPG. I had taken a little break from the server and upon my return was gifted my DM status back, and to celebrate I had found a book for monsters from one of my favorite games. (Monster Hunter World). Well anyway. I planned it as a quick little dungeon no traps no deciding directions. Just get locked in a dungeon and fight to the death. I picked two creatures that made me nervous thinking I'd TPK my group in no time flat. Lunastra and Teoastra for any fans of the series. As the encounter began I had them roll initiative and things kicked off. My cocky Barb/fighter thought this would be super easy and went right swinging at one of the monsters! Mean while A caster decided to Polymorph my second monsters. (It was at this point that I realized things werent going as planned on my part). I had made the decision to not use legendary acions until the second round just to kind of add a wow factor to the encounter. (BIG mistake). Lunaster who was the one that was not Polymorphed into a harmless kitten used a legendary action to move without drawing an attack from the barb/fighter. moved towards her partner. Now fast forward to round 2. a gnome artificer grabs the kitten and shoves it in a bag of holding…. then asked if he would be allowed to throw the bag at the Lunastra….. meanwhile If i would Allow the caster to mage hand a porthole into the bag as it reaches the dragon. (now i know according to rules and such I should say no at this point, but the idea is just too damn good!) I allow it with some checks that have to be rolled…. they all pass with flying colors! Now i start panic. and as my elder dragons get sucked to the astral sea. I start to cry and admit defeat. Now when anyone joins any of my encounters that I set up they laugh and inform me that they are going to be bring bags of holding.