I cannot for the life of me remember the name of this effect. It was an incredibly simple method with a fully gaffed deck, practically self-working.

The deck is spread to show all the cards are different. The deck is then closed and the back of one card is touched and shown as the selection. The deck is shuffled and one card is presented. The card is incorrect. A color change is done and is shown to be the correct card. Let’s say it was the 3 of Hearts. Then the deck is spread out face up again and every card is shown to be the 3 of Hearts. No deck switch. The method, again, was incredibly simple but only if you’re familiar with gaffed cards which excludes most laymen.

Does anyone know the name of this effect?

Sounds like a Svengali Deck, no?

POSITION IMPOSSIBLE By Brent Braun

I suppose you might be able to do this with Position Impossible, but that effect is sold as an ACAAN-style effect. I guess any similarly-gaffed deck could be used. But this sounds more like Dani DaOrtiz' The Mirage, but not quite the same.

Destiny Deck?
https://www.card-shark.de/TheDestinyDeck/

Can be done with a Mind Power Deck.

My mental lapse. Searching memory for the correct name.

It’s the one with normal indexes at one end, force card indexes on the other. Often used for a very clean force plus ACAAN.

Got it. It is the Mastermind Deck.