Invasion

An Invasion is an in-game event that was added in the August 1st, 2014 update, along with Bandits.

Description
Unlike bandits, invasions don't come without warning; the first sign that you're going to be in for a fight is a low-pitched horn blow, and a message in chat stating "The [Faction] have launched an Invasion!". These invasions can occur in any biome, from Rohan to Gondor to the Lone-Lands, but are most dangerous in "wilderness" biomes -- that is, Wilderland, Lone-Lands, and the associated biomes.

When the invasion begins, a large sword will appear floating over the ground, surrounded by flames that don't give off any light. When this happens, you have two options; take cover and run, or kill all the enemy units. If you choose the latter, beware that an invasion could result in 25 or more enemy units appearing within a very short period of time, but also note that slaying all the enemy units could result in you getting a lot of Alignment with the faction you're fighting.

The units spawned in an invasion can include any units the enemy has at its disposal -- for example, a Rohirric invasion of the Uruk Highlands will contain mostly mounted Rohirrim and Archers, whereas an Angmar invasion of the Lone-Lands will contain all possible units allied with Angmar.

The spawning of enemy units will end after about a minute or two and the sword will then vanish.