I thought it was Len wiseman
Kevin Greaveoux co-created it, (and probably created it) and in one of his in one of his interviews, he has mentioned that the Underworld story was a allegory to his personal relationships.
EDIT: from his Wiki page:
Underworld was Grevioux's first produced writing credit. He came up with the original concept/premise and wrote the original screenplay along with director
Len Wiseman, launching the
Underworld franchise. He appears in the film himself as a lycan,
Raze, and returned as the character in the Underworld prequel
Underworld: Rise of the Lycans, which he will also be adapting into a comic book mini-series. He also expressed an interest in telling more Underworld stories in comic form if this proves successful.
[1]
Using
Romeo and Juliet as an
archetype, Grevioux based
Underworld on his experiences with
interracial dating and the tension that it often causes. He also brought a scientific element to the world of vampires and werewolves by basing
vampirism and
lycanthropy on a
viral mutagen rather than the
mysticism typically associated with these two
mythic creatures.