Hilary Raintree is a vampire in the era of the Grendel Khan. There are many vampires at this point, but most of them are in hiding. Pellon Cross and the other leadership are in the System of VEGAS, and would like to infiltrate the casinos, but they can't get past the blood tests that are done on everyone who enters.
Hilary, however, has one unusual characteristic. Unlike every other vampire, she bleeds when she is cut, because she was a hemophiliac when she was alive.
Cross gives her a mission: to get inside the casino and turn as many people into vampires as possible. She crawls past the outer guards in her animal form, as a worm. Then she goes past the inside guards, who test her for blood, and she passes.
Then she slaughters innumerable people inside: cooks, guards, casino guests. She makes it to the security control center and kills all of them too. All her victims turn into vampires, and soon almost everyone in the casino is afflicted. Then Cross marches in and takes over, making the casino his new base of operations.
Hilary drinks so much blood during the attack that she dies of bloating.
Hilary presumably has the standard powers of a vampire in the world of Grendel, which include enhanced speed, strength, and durability, as well as the ability to turn into a worm. She also presumably has their standard weaknesses: water causes them pain and fire can destroy them.
She is dangerous enough to kill numerous armed guards simultaneously.