Back before Precedence lost the license, Kevin Tewart (line manager for B5CCG) made a comment that he saw the next expansion after the ranger expansion, Anla'Shok, to be "The Vorlons." He didn't expand much, merely to indicate that he saw it as parallel to The Shadows.
Consequently, after I finished Anla'shok, it seemed reasonable to me to explore this notion. The expansion was designed to be draftable and developed using contemporary notions of how CCGs work. It is, therefore, a relatively self-contained expansion.
There are no new rules for the Vorlons expansion. The expansion was designed to be played under Engine Rules therefore it takes full advantage of new terminology, specifically tainted and allergic as concepts.
Two new keywords were added: ephemeral and untouchable but they had explanatory text attached.
Do recall that these are merely works of fan fiction in homage of a great show and have no 'official' status.