User blog comment:Deedlit11/Bignum Bakeoff appetizer/@comment-2602:306:CD4B:DFF0:3C15:A59C:3CE:CC83-20171106235325

Ruby, 109 characters

f=->a,n{c,d,e=a;a==0?n:a!=[*a]?a-1:e==1?c:(x=c,d,f[e,n];d<0?x:[x,d-1,x])};h=0,k=9,0;(h=f[h,k*=k])while h!=1;k

Should be something like H&subφ&sub8(0);;(81), give or take a bit.