They Came for Blood is a desk simulation game. Your goal is to make it through the month and make enough income to afford rent in your apartment. Greet donors at the blood bank, look at their ID to determine if they're good to donate blood, and either let them inside, or reject them for not meeting the requirements. Sometimes vampires try to slip in, which can spell disaster for the hospital staff. Let in too many vampires, and the staff will be picked off one by one until you're the only one left to suck dry. There are multiple endings, and several decisions to make while sorting donors as quickly as possible. They Came for Blood has a branching save file system, in case you've realized you've made a horrible mistake and need to turn back the clock.