The Collaborative International Dictionary of English v.0.48:

Mediatorial \Me`di*a*to"ri*al\, a.
   Of or pertaining to a mediator, or to mediation; mediatory;
   as, a mediatorial office. -- Me`di*a*to"ri*al*ly, adv.
   [1913 Webster]

         My measures were . . . healing and mediatorial.
                                                  --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mediatorial
    adj 1: of or relating to a mediator or the duties of a mediator