The most recent set to explore nightmares fully was Duskmourn: House of Horror with its Fear of cards, which appeared in all five colors as enchantment creatures that Valgavoth created by manifesting the fears of survivors
The equipment that exists at more than 4 cost can have great effects but either the equip cost is prohibitive to where youre basically playing a 5 cost card that says Target Steiner gets +1/+1 or youre holding it in your hand until you have a cost cheater on the battlefield which may or may not happen
I suspect it is still garbage, but in some bizarre Storm style deck, maybe the counters go up fast enough
Ozolith, the Shattered Spire Ozolith, the Shattered Spire sits in that special place as a payoff for playing counters, because it doubles them, and a distributor, with the activated ability
Sacrifice after III.) I, II Prevent all damage that would be dealt to creatures you control this turn
Jace, Memory AdeptJace, Memory Adept has a loyalty ability that lets you mill ten cards