The Collaborative International Dictionary of English v.0.48:

Internecine \In`ter*ne"cine\, a. [L. internecinus deadly,
   murderous, fr. internecare to kill, to slaughter; inter
   between + necare to kill; akin to Gr. ? dead. See
   Necromancy.]
   1. Involving, or accompanied by, mutual slaughter; mutually
      destructive.
      [1913 Webster]

            Internecine quarrels, horrible tumults, stain the
            streets with blood.                   --Motley.
      [1913 Webster]

   2. Of or pertaining to internal conflicts within a group; as,
      internecine quarrels.

   Syn: intramural.
        [PJC]

WordNet (r) 3.0 (2006):

internecine
    adj 1: (of conflict) within a group or organization; "an
           internecine feud among proxy holders"
    2: characterized by bloodshed and carnage for both sides;
       "internecine war"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "internecine":
   baneful, brutal, calamitous, cataclysmal, cataclysmic,
   catastrophic, consuming, consumptive, deadly, death-bringing,
   deathful, deathly, demolishing, demolitionary, depredatory,
   desolating, destroying, destructive, devastating, disastrous,
   doomful, fatal, fateful, feral, fratricidal, killing, lethal,
   malign, malignant, mortal, nihilist, nihilistic, pernicious,
   ravaging, ruining, ruinous, savage, self-destructive,
   subversionary, subversive, suicidal, vandalic, vandalish,
   vandalistic, virulent, wasteful, wasting, withering