The dreaded black triangle of whatever it is....

Long story short… :B

Better winterization, better tank flushing practice is the answer… but lacking those skills up front, a poop pyramid is what you might end up with…

Fill the tank with water and some digester as some have suggested, cross your fingers and hope for a few days… agitation might help to… drain and do it again…

in the end it might take high pressure stimulation or some mechanical form of stimulation to break it up…

good luck…