The Collaborative International Dictionary of English v.0.48:

Preludial \Pre*lud"i*al\, a.
   Of or pertaining to a prelude; of the nature of a prelude;
   introductory. [R.]
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "preludial":
   antecedent, anterior, chief, exordial, first, foremost, headmost,
   inaugural, inductive, initiatory, introductory, leading, precedent,
   preceding, precessional, precursory, prefatory, prefixed, prelim,
   preliminary, prelusive, preparative, preparatory, prepositive,
   prevenient, prior, proemial, propaedeutic