Vascular brain lesions have increased risk of intracranial bleeding and, therefore, present a challenge to neurosurgeons attempting surgical resection. Such tumors may first be embolized prior to surgical excision in order to reduce the risk of bleeding. Onyx, an ethylene vinyl alcohol copolymer, is one of many embolic agents available to accomplish this task. Onyx has a dark black color that can be appreciated on both macroscopic (i.e. naked eye) and microscopic assessment. For this patient, multiple embolization courses were undertaken in the effort to embolize an unusually large arteriovenous malformation (AVM). The intraluminal black material in this image is in the early stages of resorption; it is presumed to originate from earlier embolization sessions more remote from the time of resection, which thereby allowed time for activated macrophages to coalesce into the resorptive multinucleated giant cells pictured here.