The Global Effects promo are a set of cards that are shuffled into the Darkness Spreads deck at the beginning of the game. As they are revealed, they will have various effects on the game, with 5 of the effects being good effects and 5 being bad.
Global Effects cards are also included in the Dragon Expansion as well as Hero Expansions 1, 2, and 3. There are currently 37 unique Global Effects cards, 19 good and 18 bad (1 bad card is duplicated in the dragon expansion), that are randomly inserted into the expansions and card packs available via BGG. The expansions contain 6 cards, 3 good and 3 bad, while the BGG promo packs contain 10, 5 good and 5 bad.
You are able to combine Global Effects from the expansions/promos however you would like, affording a great variety from game to game.