A blood jar is just as its name suggests - a jar of blood. It is one of the two methods that can be used to keep The Fiend at bay. In order for a blood jar to be made, the user must add three drops of their own blood, as well as three drops of the blood of one of The Fiend's children, and mix them. As long as the blood jar is kept on the user's person, The Fiend cannot approach them. In addition, anyone near them is also under the jar's protection. As Shown in Rage of the Fallen, if the jar is cracked, then The Fiend will be able to get closer and closer to the user as blood leaks from the jar. Eventually, the jar will fail, and the refuge it provides from The Fiend will have been relinquished.

The blood jar was first introduced in The Spook's Mistake, when Alice Deane suggests using one in order to keep The Fiend away from Thomas James Ward. Tom declines, however, because using this method is basically the same as using the dark. In The Spook's Sacrifice, the blood jar is finally put into use when Tom sells his soul to The Fiend. The pact stated that in three days time, The Fiend would come to collect Tom's soul in exchange for great advantages over The Ordeen the present. However, when The Fiend comes to collect Tom's soul, Alice thrusts the jar into his hands. Later, it is revealed that Alice stole some drops of Tom's blood when he was knocked unconcious by falling rocks during a cave-in earlier in the book. Tom exclaims that Alice must stay close to him from then on, because if The Fiend ever got the chance, he would kill her in revenge, and subject her to eternal torment in The Dark.