The Collaborative International Dictionary of English v.0.48:

Obsidional \Ob*sid"i*o*nal\, a. [L. obsidionalis, from obsidio a
   siege, obsidere to besiege: cf. F. obsidional. See Obsess.]
   Of or pertaining to a siege.
   [1913 Webster]

   Obsidional crown (Rom. Antiq.), a crown bestowed upon a
      general who raised the siege of a beleaguered place, or
      upon one who held out against a siege.
      [1913 Webster]