The Collaborative International Dictionary of English v.0.48:

Exitial \Ex*i"tial\, Exitious \Ex*i"tious\, a. [L. exitialis,
   exitious, fr. exitium a going out, a going to naught, i. e.,
   ruin, fr.exire to go out: cf. F. exitial.]
   Destructive; fatal. [Obs.] "Exitial fevers." --Harvey.
   [1913 Webster]