EMB-145 Captain reported during the descent encountering severe turbulence resulting in uncontrolled aircraft movement; the flight crew regained control of the aircraft and landed safely at their destination.

Narrative

During the descent into ZZZ1 on the ZZZZZ RNAV arrival; the crew was initially instructed by ATC to 'descend via ZZZZZ; except maintain 310 knots or better until ZZZZZ1.' Shortly afterward; ATC amended the clearance to 'resume published speeds' beginning at ZZZZZ2 intersection.While complying with the amended clearance and descending from FL240 to FL200; the aircraft encountered severe turbulence prior to ZZZZZ2. The event involved abrupt; uncontrolled aircraft movement; including: Roll: Uncommanded left roll up to 20° bank Pitch/Attitude: Sudden pitch changes Airspeed Variation: ±20 knots IAS Vertical Speed Variation: ±500 FPM Autopilot: Disengaged during the eventThe turbulence episode lasted approximately 20 seconds; after which positive control of the aircraft was regained. ATC was notified immediately.Outcome: No injuries to passengers or crew. Aircraft continued the arrival and landed safely at ZZZ1. After landing; maintenance was notified; and the discrepancy was documented in the aircraft logbook.Contributing Factors: Clear air turbulence in descent segment of ZZZZZ arrival. Speed reduction in compliance with amended ATC instructions near ZZZZZ2 intersection likely coincided with area of turbulence.Corrective Actions: Flight crew reported event to ATC and maintenance. Maintenance logbook entry initiated for autopilot disconnect during turbulence encounter.