Air Carrier First Offer reported encountering severe turbulence during descent resulting in uncommanded pitch and bank changes. The Flight Crew regained control of the aircraft and landed safely at their destination.

Narrative

On descent into ZZZ on the ZZZZZ arrival around the ZZZZZ intersection we encountered severe turbulence. We were aware on turbulent conditions that existed from multiple reports of turbulence along with WSI (Weather App) and SkyPath showing turbulent conditions on the descent into ZZZ. Along with the windy conditions on the ground and the storm clouds over the area. We prepared for it by making an announcement about it and having the flight attendants clean up and be seated for the descent. The ride down was mostly light chop until about FL210 when the aircraft suddenly began to experience uncommanded pitch and bank changes. We were at 280kts and the autopilot was on. It was struggling to correct the deviations. The aircraft seemed to be in an updraft as the descent started to trend into a climb at one point. The whole event seemed to last a while but in my guess was only about a minute. We declared the event to ATC and checked on the flight attendants and pax (Passengers). All was ok and no apparent damage to the aircraft. The rest of the flight was just light turbulence with a moderate bump. We landed and wrote up a severe turbulence encounter as per the definition in the FOM. The tech ops personnel met the aircraft and we described what had happened. They asked a few questions about airspeed; altitude; and configuration. The flight attendants also remarked on how incredibly bumpy it was and had they not been seated it would have tossed them to the floor. Luckily they were seated and the passengers too. I attribute the successful mitigation of injuries or damage to the tools we have available in the cockpit. We knew about when we would start getting turbulence and gave ample time to prepare.