The Bleeding Tree is an entity associated with the Slender Man, originating in the Blogosphere canon. Although a major part of the Blogosphere's story arc, the Bleeding Tree gained little traction with the rest of the Mythos, and is usually more associated with the The Fear Mythos.
The Bleeding Tree is a sentient and malevolent tree that resides within The Path of Black Leaves . Like all trees within that realm, its leaves are black. Additionally, it is often described as larger than its fellow trees, a large human face growing on its trunk, corpses hanging from its branches, and is constantly bleeding. It is not known if the Tree is an aspect of the Slender Man or a separate entity, although it did seem to have an opposing agenda to a prevalent proxy. It seems to have telepathic abilities, broadcasting itself as a hallucination to intended targets. It also has an ability to "infect" victims, which entails the Tree growing inside the victim until it rips them apart.
The history of the Bleeding Tree mostly revolves around other figures in the Mythos (more specifically, the Blogosphere), due to its passive nature.
The Bleeding Tree first appeared to the blogger Zero in The Last Refuge of a Dangerous Man, haunting him since a failed attempt to defeat the Slender Man at the solstice. There was little information on it, and most other bloggers presumed it to be another hallucination caused by the Slender Man. The blogger Robert of White Elephants learned from Zero how to find the Bleeding Tree, and took samples from it. Using a piece of the Bleeding Tree, Robert attempted to kill the notorious Proxy Redlight.
Redlight survived the encounter, and provided a fellow Proxy with syringes that cause those injected to have trees grow out of them, killing them. It is unknown, but implied, that these syringes had some sort of conncetion to the Bleeding Tree. Enough poison was kept for over a hundred victims. It was ultimately revealed the use of the poison caused damage to the fabric of reality (referred to as "The Veil"), and that the full contents of the syringes was able to destroy the world.
Redlight revealed that he was infected by the Bleeding Tree since his encounter with Robert. His gambit with the syringes was a threat to force the bloggers to help him. Redlight's ability to shift his consciousness between bodies was useless, as the Tree had managed to infect all of his backup bodies. Redlight warned that the Bleeding Tree was the real threat, and that the bloggers had to stop even if it meant curing their enemy. The bloggers realized this, but chose to ensure Redlight's death as well. AmalSage managed to block off Redlight's ability to shift-body, so that the Bleeding Tree would kill him in his current body. This turned out to be exactly what Redlight wanted, as the severed connection meant the Bleeding Tree couldn't track him, so he was able to possess an uninfected body, though he would ultimately be killed in that form.
This section's source of information does not discuss any further the infected bodies/syringes plotline. If anyone knows how this was resolved, please add it.