I believe the invasion forces are/were triggered both (1) by the developers, and (2) by SL or Minax corrupting a Fel faction town's sigil.
Once an invasion is triggered, the townspeople on the Trammel side will become corrupted either by Hatred, Fear, or Falsehood. If your character is in factions and kills corrupted townsfolk, the faction character receives silver in his/her backpack for each kill (usually 5-6 silver, although I think larger amounts are possible on a random basis). Some townsfolk also drop gold as loot, but it isn't much. Most drop nothing but clothing and other odds and ends. Killing them causes a loss of karma. The Falsehood-infected townsfolk also steal uninsured items, e.g., bandages. You can get the item back by killing the thief and/or when the invasion is completely stopped.
Various monsters also spawn in each Trammel town when the invasion starts in that town: [Note: On Siege and Mugen, the invasion happens in the town in Fel.]
- Britain - Dragons
- Magincia - Undead
- Minoc - Plague
- Moonglow - Void (e.g., ethereal warriors)
- Skara Brae - Elementals
- Trinsic - Orcs
- Vesper - Ophidians
- Yew - Demons
The invasion of a town is supposed to stop when either (1) the invasion forces (i.e., the monsters) are all defeated; or (2) when either the COM or TB faction is able to seize, corrupt, and return the town's sigil back to town on the Fel side.
I believe that the invasions started out with low-level forces first. As the towns were re-invaded, or forces were not defeated (i.e., the invasion moved onto its next stage), higher-ranking forces started to appear. If the invasion of a town is able to proceed to its final stage, a crimson dragon appears. If the crimson dragon is defeated, then the town will return to normal. [After seeing so many reports of towns with corrupted townsfolk and no crimson dragon, I think that what has happened is that these towns have never had a final stage invasion. Perhaps if TB or COM can control those towns once more and then SL or Minax can take control again, they will see a stronger invasion on the next go-round.]
Only three towns at a time can have an active invasion. If more than three towns are under the control of SL or Minax, the additional towns will be queued up for invasion after another town's invasion has ended.
At some point, we expect to see platinum dragons appear. All we know about them at this point is that they are enemies of the crimson dragons, and like the crimson dragons, their presence will cause tamed dragons to go wild.
As far as loot goes, killing invasion monsters or army forces with a faction character is rewarded with silver that pops in your character's backpack. If your faction character is killed by an invasion monster, army member, or a corrupted town NPC, your faction character will go into skill loss for 20 minutes. Faction characters earn no silver from killing the crimson dragon, but also don't suffer skill loss if they are killed by one. Nonfaction characters earn no silver and do not have to be concerned about the possibility of skill loss if they choose to participate in fighting invasion forces.
If you kill an army member of the invasion forces, there is a chance that a spellbook in an unusual color or a weapon will pop into your character's backpack. The properties of these items depend on the rank of the army member killed, with better properties on the items that come from killing higher-ranking army members.
More info here:
http://www.uoherald.com/news/news.php?newsid=454 .
