In my last campaign, the wizard died (he got better) and his staff of the magi was stolen by a big bad and given to one of its lieutenants (a CR20 rogue with purple worm poison). Later, the evil rogue absolutely frustrated the group with her Use Magic Device feature to attune to the staff and combinations of invisibility, Hide, and move. She had them running all over different parts of the inside and outside of a ziggurat, splitting the party, and sniping when there was a free moment. She blew the monk off of the ziggurat with telekinesis, poisoned the wizard with her crossbow while hidden, and had everyone slowly losing health and resources. When they started trying to corral her with a clever use of locate object, she Move, Dash, Dashed to the next level of the ziggurat, turned off invisibility, and waited. The wizard flew up to her on his broom and failed to hold her with hold monster. She executed her final order ("Kill the wizard") by smashing the staff over her knee, vaporizing herself in the process. The wizard made his saving through so was not killed outright, but he was unconscious with 1 failed death saving throw from falling to the ground. The party saved him, but it was a harrowing experience.
