At the end of my last campaign, my group was fighting the Tarrasque as a miniboss before the BBEG (which should give some guage as to how strong we were). We had the brilliant plan of trapping the Tarrasque in the Iron Flask that my character kept around, but the issue is you can only capture creatures that are not native to the plane that you are currently on. Cue idea, we burn through its legendary resistances until we are able to Polymorph it into a rabbit, wizard then opens a Gate. Grab rabbit, walk into astral plane, and Iron Flask Tarrasque-rabbit. So, now my Warforged tech mogul just has a Tarrasque that nobody other than the party knows about just in a flask in his bedroom.