Speaking of which, now that I think about the d12 is nearly never used. Lots use D4 (mage/sorc hp, magic missile, dagger), d6 (fireball, short sword), d8 (long sword, monster hp), d10 (percentile, stabilization check), d20 (duh). d12 for some few weapons, but that's about it.
I like to use loaded d% for that. CR 90 encounters against a first level party a fun. Though if they manage to complete it they get enough experience to straight to epic. Do not pass 10 do not collect 200 magic items.
Ancient x46 Force Dragon that has had all its defenses lowered, been knocked out by, and been reduced to 1 HP by a party of lv. 91ers that just came by?