Center Controller reported an aircraft encountered icing; their airspeed indicator failed; and they descended below the minimum safe altitude.

Narrative

Aircraft X was originally at 6000 ft. flying to 8W2. CRW approach called to approve the aircraft at 6000 ft. because of icing. I approved it and told the controller that Aircraft X would have to climb to 6500 ft. at some point because of my MIA [Minimum IFR Altitude] or turn to the right to stay in the 6000 ft. MIA shelf. When Aircraft X came over we were having communication issues. I had to relay through Aircraft Y and turned Aircraft X to the right so he could stay at 6000 ft.. Eventually I was able to switch Aircraft X to another frequency and communicate with him directly. When the aircraft was ready to climb he requested higher and direct on course. He remained at 7000 ft. for about 40 miles and then proceeded to ask when he could have lower. I told the pilot that it would be at least another 20 miles before I could descend him to 6100 ft; but I could offer him 6500 ft. at that time. Aircraft X decided to accept 6500 ft. so I descended the aircraft.About a minute or two after leveling off at 6500 ft; the pilot said that they weren't going to be able to maintain altitude due to severe icing and that they were losing airspeed. I was not able to understand everything he said other than he couldn't maintain altitude and that they were losing airspeed. It was not until I reviewed the [Radar display replay] that I heard he reported severe icing. I asked if he wanted to climb back up not realizing that he couldn't maintain airspeed due to severe icing. When I asked what I could do to help; he told me he was stalling. I tried to communicate with Aircraft X; but the radios were coming in broken. I believe he was trying to get the aircraft under control. When I was finally able to reach him again; Aircraft X was at 5900 ft. and descending. The pilot told me he lost his airspeed indicator and was descending to VMC. I told him that the MIA was 6500 ft. and advised him to turn to the right. MIA to the south of his location is 6000 ft. He did not want to turn and said he was just VMC now. I offered him any type of help he wanted again; but I got no response. The radio communications were spotty for what felt like a minute. Eventually I was able to talk to Aircraft X again and he said he was VMC at 4500 ft; that he lost his airspeed indicator; and that he wanted to land as soon as possible. I told him to standby and I pulled up the emergency airports. I knew HSP was a good option; but I wanted to make sure it was the best option. Once I confirmed that HSP was the best option I told the pilot about it. I told Aircraft X that HSP was 20 miles south of his current location.The pilot then told me he would head direct to the airport and remain at 4300 ft. below the cloud layer at 4500 ft. and judge airspeed. I once advised him again that the MIA was 6500 ft. and he responded by saying they would use caution. A few seconds later; I finally asked if he was [requesting priority handling] and he said yes. I then proceeded to tell my supervisor. I got the information my supervisor requested and then I read the HSP weather to the pilot. I looked at the NOTAMs; but in my opinion they did not apply. Fuel not available and obstruction tower lights out of service. About 10 miles from the field; Aircraft X got their airspeed indicator back and told me about the severe icing. That was the first time I heard him report it. I called the airport 5 miles out and told the pilot to report the field in sight. He reported the field in sight 3 miles out and canceled IFR. My supervisor called HSP and verified that Aircraft X landed safely.There were no icing reports in my sector prior to the event. No weather was depicted on my sector. The only weather depicted was in CRW approach. The last tops report was at 10000 ft. at least 10 minutes prior to the emergency. The only thing I can think of is I could have solicited PIREPS once CRW approach advised me there was icing outside of my area.