The Great Vacker Hurlfest, as Keefe dubbed it, occurred when Fitz was stabbed in the chest by an arthropleura called by Councillor Clarette during the Black Swan's raid on Exile. Physic and Sophie then placed a yellow, sludgy medicine under his tongue that was able to get the venom out of his system. Unfortunately, this medicine forced him to throw up the black venom. When Fitz vomited up the venom, Biana barfed, causing Della to do the same, "and repeat!" as Keefe said.
“I thought I should be a good friend and check on Fitz. Do not go in there. It's pretty much the Great Vacker Hurlfest. Every time Fitz spews up black gunk, it makes Biana barf, and then Della loses it—and repeat!”