They Hunger: Lost Souls. For Half-Life 2.
If you're a fan of both Half-life and Horror games there is no way you wouldn't have played the original "They Hunger" mod back in 1999. It was considered one of the most scariest games of all times. Some consider it much scarier than Dead Space and Amnesia. It was followed up by two sequels but they never beat the original's atmosphere.
After 5 years, it was finally going to have a faithful reboot to the franchise with the commercial mod "They Hunger: Lost Souls" by the same guy that made the original, but the mod author died recently two years back and thus the mod is considered dead. The beta testers said it was going to be really awesome... What a shame.