To make and use Rain of blood you need to combine a low tier hydrosophist and a low tier necromancy books together via crafting. To use the skill you'll need to have a point in each of these schools.
To solve the historian issue without resorting to Rain blood:
If you are an elf, teleport or jump next to the historian and flesh sacrifice a few times around him to produce a few pools of blood. Eventually the blood will touch him.
If you don't have an elf in your squad, you can use the shapeshifting mask and temporarily transform into one. You'll eventually obtain this item f you explored the island, i'm not gonna say where exactly it is but i'm sure you already stumbled onto it.
If you have more party members you can place them next to him and hit your own teammates, when you start damaging the health your characters will bleed until there is enough to the ground.
