Clarification, Lord British and the Council of Wizards are the abandoners.
After Trinsic was conquered by Juo'nar, Malabelle, and Minax,
Nystul found a way to the "Trammel Facet" and the entire government of Britannia fled its original home -- abandoning it to Minax. Hence, abandoned lands. Many of those of us around at the time thought this puzzling and, frankly, cowardly.
Here is a story from the time of The Abandonment.
http://www.geocities.ws/myqsa/xylan.html
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The current invasions continue. The number of captains in an invasion seems to be based upon the number of defenders, and which stage the invasion is in. An invasion proceeds in several stages. At the end of each stage the beacon [lighthouse] may be attacked for a short time.
Each invasion seems to be themed, that is, say, all captains are Masters of Dragons.
The beacon [lighthouse] may be defended by a contrasting type of minion.
It seems that each Captain carries one Minax-crest dark red hued item, which one of his or her attackers will find. (This is typical. I have also heard of two items being found, or none.) Each attacker of the Beacon will also find a Minax-crest item. Uncommon but still possible, killing a minion can yield a Minax-crest item.
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Each city in Trammel Facet has been attacked at least once.
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I have gained 42 Minax crest items to date, about one per hour. My skills are mostly on the defense side rather than on attacking. I hear that people who are better attackers (mage tamers, mage spellweavers, archers with velocity/effect weapons) are getting a better yield -- but then, the grass is always greener.