Bloodwood trees are a genuinely horrible creation. Their very existence a wound in the decency of the world, because this tree grows only in places steeped in violence and death.
The biggest bloodwood trees tower over battlefields. Sinewy spiderlike branches cast shade over the homes of serial killers, and you should never trust anyone that can grow one in their garden.
Leech-like roots soak up the suffering, sorrow and cruelty that has seeped into the soil, and then converts that misery into a sticky red sap of raw emotion. This sap can be used to amplify feelings, to convey anger, fear or hatred, across a much greater range.
The dragonkin Lacrussa was very fond of this medium, and used it’s power to shackle dragons to her will.