wedll I think it is something to do with dragons ... but I am not sure which is the odd one out ......
War of Wrath - Earendil slays Ancalagon,
Battle of Five Armies ...Smaug's death is the catalyst
Nirnaeth - Galurung causes devastation but survives
Battle of the Pellenor Fields - the Nazgul's steed could be termed a dragon I think - monstrous winged creature without feathers.......
So Pellenor could be the odd one out if you don't class the feel beast as a dragon
oo - no dragon
Five Armies OOO- caused by dragon not featuring a dragon
Nirnaeth OOO Dragon lives to fight another day....