While this looks like a wierd card, this is a god-sent card for the defensive player. When you attack (a realm, not any other card), you play this event when you have a champion you don't mind losing in combat. That champion, and the champion opposing it (assuming the defender didn't win by instant defeat, which is something that will prevent Kamikaze! from being played) both will be sent to the discard pile. 

This can even be improved if you use a champion in battle that is immune to events, which cause the immune champion just to return to their pool (this can also backfire and save a defending champion who is immune). This result is often worth it when you know the opponent will put out their best defender.

In addition, you get to raze the attacked realm, which can be very useful against an opponent on the cusp of victory. Not something for the timid at heart, or those needing to keep all of their champions, but it can be worth it if you don't expect you opponent to be able to prevent the razing.

This is a good card for major defensive decks, especially those that bring champions back from the discard pile (like many cleric themed decks). As always, and opponent used to such tactics can either looks for more realms and champions immune to events, use standard event stoppers, or carry realm rebuilders.