Blood Vestibule is the entrance to the four hunting areas of the Rotten Blood Quest: Jaded Roots, Putrefactory, Darklight Core, and Gloom Pillars. According to A Bloodshade, that is part of the "holy realm of blood", belonging to the sanguine lord, the blood god.
A Bloodshade informs players that a spawn of the sanguine lord, Bakragore, has grown in power in the area, giving birth to two sets of twins: Murcion and Ichgahal, and Vemiath and Chagorz. They are corrupting the realm, which is a mockery of its blood god's grandeur, and must be exterminated to cleanse the realm and let it heal.