EMB-145 Captain reported loss of aircraft control for a short period of time when they entered an area of severe turbulence.

Narrative

We were in cruise at 13;000 feet; coming from ZZZ; and about to begin our descent into ZZZ1. There had been a lot of bad weather in the general area all day. We passed through some clouds that appeared to have tops of about 15;000 feet. The radar was not indicating anything but very light scattered precipitation. As we entered the clouds; we encountered severe turbulence. The aircraft was thrown down about 400 feet instantly with about 20 degrees of bank to the left. Speed increased towards red line; 320 knots; and I deployed the spoilers. As we climbed back up towards 13;000 feet the aircraft was repeatedly thrown down to a lower altitude. After about a minute; the turbulence subsided to light turbulence and we climbed back to our assigned altitude of 13;000 feet and on course. We reported the severe turbulence and the deviation to ATC and continued on to our destination.Suggestion: Possibly request a deviation around clouds when widespread bad weather has been occurring for a prolonged period of time.