internecion

from The Century Dictionary.

• noun General slaughter or destruction.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete Mutual slaughter or destruction; massacre.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun destruction, massacre ("laughter. — That natural propension of self-love, and natural principle of self-preservation, will necessarily break out into wars and internecions" Hale)
• noun Mutual destruction (less common)