"One Dangerous Zombie" is a feature in the Resident Evil remake which sees a zombified and essentially invulnerable Forest Speyer wandering the mansion. It is unlocked in the 2002 GameCube version by completing Jill Valentine and Chris Redfield's scenarios twice each (playing "Once Again..." for both characters). In the HD remaster, it is unlocked by completing Chris and Jill's scenarios once each on Normal or Hard difficulty. Rather than being a separate mode, this feature becomes a permanent part of each "Once Again..." playthrough of the game and cannot be turned off.
Forest will be present in various locations throughout the mansion during the first section of the game. His appearances are not randomized and thus he can be anticipated. Forest dons a vest with multiple grenades strapped to his bandolier; even when off-screen, one can confirm he is present in the room by the rattling of his grenades and his shuffling feet. He runs faster than other zombies and lunges further forward than others when attempting to grab the player. When Forest appears, he usually (though not always) replaces all zombies that would otherwise be in the room, and once he leaves the room they do not return. Forest only appears early in the game, and only in the mansion; once the player returns from the Dormitory with the Helmet Key and the Hunters make their appearance, Forest will no longer appear.
Forest cannot be killed without also killing the player and resulting in a game over, due to the grenades that are strapped to him detonating when attacked. In addition, defense items such as the dagger, Flash Grenade, and stun gun cannot be used on Forest, so the player must take damage if grabbed by him. Therefore, players must do their best to avoid him. Players should consider unequipping their weapon when running from him as this will make Jill and Chris run faster.
- Tea room (near Kenneth J. Sullivan's corpse with his back to the Birdcage corridor)
- Dining hall (replaces the zombie on the second floor)
- 2F west staircase (replaces the zombie that arrives through the door from the West terrace when the player returns to pick up the serum for Richard Aiken)
- Small dining room (replaces the zombie hiding behind the cabinet near the Musical Score)
- Corridor to the back entrance (replaces the zombie near the door leading to the passage to the shed)
- west terrace
- The tea room is the only room in which Forest is unaware of the player until they approach in front of him.
- Forest does not appear in the East terrace – where his corpse would normally appear – at all while this feature is active.