Chainer, Dementia Master is a rare Legendary Creature — Human Minion card from Dominaria Remastered (DMR), collector number 299. Current market price: $0.33 (foil $0.42), TCGplayer market prices via Scryfall.
All Nightmares get +1/+1. {B}{B}{B}, Pay 3 life: Put target creature card from a graveyard onto the battlefield under your control. That creature is black and is a Nightmare in addition to its other creature types. When Chainer leaves the battlefield, exile all Nightmares.