Pilot reported entering severe icing during cruise. Pilot reported descending to lower altitude and continued to destination.

Narrative

I was the PIC for a ZZZZ to PASC Part 91 repositioning leg.I filed an IFR flight plan to PASC at 10;000' via THAWD; with a plan for the block 10;000'-12;000' if needed to avoid prolonged icing on our planned route. The NWS AAWU (Alaska Aviation Weather Unit) icing forecast indicated isolated moderate icing over the White Mountains and Yukon Flats from the altitudes 030-120 during our flight.We started to pick up ice at 8;000' (light mixed).On the climb to 10;000' I realized that wouldn't keep us from continuous icing conditions; asked/received for block 10;000'-12;000'.At 12;000' icing persisted (moderate mixed)...asked/received block 10;000'-14;000'.Icing increased.Increased power to maximum Torque/ITT (Interstage Turbine Temperature) to maintain airspeed above icing limits.At 13;300' unable positive climb within icing airspeed limits; started shallow (200 fpm) descent to maintain icing airspeed.Stall buffet began; airspeed dropped below icing limits.Increased descent to 600fpm; no result; then 1000fpm; no result; 2000fpm halted airspeed degradation.Descended through clearance limit (10;000'); received several lower altitudes from ATC...all unable. I believe the controller said MSA was 4;000' in our immediate area.Found MEF (Maximum Elevation Figure) on the VFR charts (1;500'); made the decision that it would be the absolute floor.VMC began around 8;000'.Stall buffet ended around 4;000'.Leveled off at MEF (+7C SAT) still covered in ice...it took roughly 5 minutes for ice to fully shed from aircraft.Started shallow climb to safer altitudes...maintained VMC rest of journey.Notable additions:Unfortunately; in the descent; we continued to aggressively pick ice until approximately 8;000'...an estimated 1' had accumulated on all unprotected surfaces; to include inside engine intakes.Turning around was not a viable/safe option once buffet set in.I did not knowingly fly into severe icing...once realized; I reacted accordingly.For the entire event; all de-ice/anti-ice systems were operational and in use.ATC communications and instrument flight rules during this event were limited to the extent to meet the situation.After this event; ATC issued a CWA (Center Weather Advisory) for severe icing.