The super massive galaxy spanning Hive Fleet Ninja that attacked and destroyed the Squat Homeworlds that spanned the Center of the Milky Way Galaxy! It was such an awesome battle that every single Squat in existence was snuffed out with a word from the Hive Mind Jervis, never to show it's face in the
40k universe again.
What made the battle so extremely unusual was that not a single race was harmed except for the Squats. Even the Imperium escaped unscathed!
Then, all of a sudden, the Tyranid Hive Fleet Ninja dissappeared, leaving no trace of it's existence...not even in the Tyranid codex.
Truely, this was the greatest Battle of the Warhammer 40,000 Universe.