It is a Battle Card which causes extra losses to the loser if it is the card that wins the Battle.
It is a fairly rare card in the Battle Deck, so it is often a good gamble to use it to try and win a battle decisively even though you automatically lose the initiative.
Historically Hannibal used it, and I believe his ability in the game lets his use another card as a DE, so he will win that way (and cause extra carnage) more often than any other general.
Thanks! I saw where someone said (in another thread) that they "DE'd" Rome? Maybe they meant the Army guarding Rome/Latium.