DAY OF NIGHTMARES was Teppen's first expansion. It released in September of 2019, and added the game's first post-launch hero Jill Valentine. It also added two new abilities to the game, "Explore" which lets unit cards add certain other cards to your EX Pocket even when they aren't in your deck, and "Spillover" which lets units deal half damage to adjacent enemy units. Unlike future expansions, there is no real overarching story being told, and instead has moments from across the different games and series included in Teppen, with the main focus being on Resident Evil 3: Nemesis and Resident Evil: Operation Raccoon City.
Series and characters included as Unit Cards in this expansion
- Darkstalkers: Demitri Maximoff, Mei-Ling, Hsien-Ko, Jedah Dohma
- Devil May Cry: Trish, Angelo Credo, Agnus, Hell Vanguard
- Mega Man X: Alia, Layer, General, Metal Wing, Armor Soldier, Slash Beast, Burn Rooster, Sting Chameleon, Burn Dinorex, High Max, Vile MK-II
- Monster Hunter: Velocidrome, Agnaktor, Seregios, Kirin, Teostra, Kulu-Ya-Ku, Kushala Daora, Tigrex, Anjanath, Bazelgeuse
- Resident Evil: Leon S. Kennedy, Claire Redfield, Sherry Birkin, Ada Wong, Jill Valentine, Carlos Oliveira, Harley, Party Girl, Shona, Tweed, Willow, Dee-Ay, Bertha, Lupu, Spectre, Beltway, Four Eyes, Vector, HUNK, Licker, Nicholai Ginovaef, Sliding Worm, G-Adult, William Birkin: G1, Albert Wesker, Tyrant (T-103), Nemesis
- Street Fighter: Guile, Charlie Nash, Maki Genryusai, Zeku, T. Hawk, Blanka, Rufus, Kolin, Menat, Abigail, Vega
"DAY OF NIGHTMARES" contains examples of:
- Caps Lock: One of 3 expansions to have an All Caps title, the others being 30_MINUTES.EXE and THE BEAUTIFUL 8.
- The Cameo: A number of characters appear on Action Cards without a Unit Card to go with them. For example, Sigma appears on the "Rebirth (DON 099)" card.
- Early-Installment Weirdness: The cards added in this set tell no overarching story, and are more just documenting moments from each series.
- This is the only expansion where Legendary Unit Cards list the characters' name first, and then their title.
