There are a couple of areas to use for the Beholder Series, The Area is Suppose to be near woods, a river, and a town build on some questionable ruins. Here are some of my choices based on my research, if you wish to adapt the Beholder Series to the Mystara Setting. Starting with the Best Choice for the Trilogy, I choose known other than... Luln, Karameikos.
I chose the town of Luln, Karameikos to replace cumbert, in Eye of Pain, I place Burke's Crossing in the Achelos Estate, there it's to be a meeting place between Karameikan Merchant and the Exiled elves of Alfheim that live, with the king's permission, of course. The Elves upon hearing of your mission are sure to offer you a reward ranging from 500 to 1000 gold royals for the removal of the beholder. Since most Karameikan town, and cities are built on ruins, why not Luln? The only problem is that they don't know it was built over an ancient beholder cities predating Nithian time. Here's a hook, if you already have Baron Ludwig captured by Halfling, and got rescued to serve Old King Kol. Have it where the beholders had a treaty with the Black Eagle, in which, if he, The Baron was taken by force, the Hive Mother could take back their lands, call it an insurance policy. This is to answer the question of where was Baron von Hendricks, during the Eye of Traldar Module. Now if the module was to be played entirely during the WOTI, you can make Burke's Crossing in Achelos, as only a logging village that will turn into a meeting place for the Alfheim Avengers once the logger's are ordered to leave. In both cases, you can decide if you want to have the Unblinking Eye be a Faction of the Iron Ring working mainly in Luln or as one of it's rivals, you can have Eye of Doom take place a year after Eye of Pain and have a series of adventures where the party is hunting a criminal mastermind known only as the Faceless Man. It's would be interesting, if you were to have the Unblinking Eye as a rival to the Iron Ring, imagine having your character's having to ask the Iron Ring for help(with Minda as a possible Iron Ring Agent.) As for Eye to Eye, The story takes place completely underground in the beholder city known to them as Ith K'hinax or the 'place of spires' as they call it.
As a side note, use the tunnel leading out(1-10) of Ith K'hinax to begin the "Shards of the Day" Dungeon magazine #60 Module, where you can travel from Ith K'hinax to the old draven city of Dylvwyllyn, Ith K'Hinax's rival underdark city-state. I chose Luln, due to it's remote location from major cities like Specularum/Mirros, Kelvin, and Shireton It's 20 to 30 Miles from the Achelos Estate, a good setting for Burke's Crossing or you could say "Boris' Crossing" if you wish to make it authenticly Traladaran. My other Karameikan choice are Kelvin, with B.C.(Burke's Crossing) S. down the Vologa River, Krakatos with Beholders acting as mages who destroy prospective student who wish to take part in their 'Special Research'(For DMs looking to make a Challenging Mystery, and Adventure) or Specularum/Mirros (I don't know how you'll Eye to Eye after Eye of Doom, except immediately, Because player might try to tell King Stefan and he will order his troop to remove the threat immediately, before the PC's get a chance.)
Darokin is nation that was built, mainly of lands unclaimed by other nations that merged to become a properous kingdom of Gold, but it also attract trouble in the form of bandits, and brigands. In this Series, the Beholders don't currently care much for gold at the moment. I chose two locales for the modules, the best locale for the series in Darokin is Mar in southern Darokin, a beholder in the Malpheggi Swamp is consistant with the Video Game: Tower of Doom, let's just say for arguement's sake the the undead beholder in Eye of Pain is really the beholder slain by adventurer from the Tower of Doom Video game. Have B.C. across the Streel River with the Beholder lair near the Malpheggi Swamp. For Eye of Doom, you can have the Unblinking eye as an ally or rival to the Bandit Kingdom(Karameikos has the Iron Ring, Glantri has the Unseen hand, and Darokin has the Bandit Kingdom, a merchant enemy is a brigand.) For Eye to Eye, you can have the tunnel of Ith K'hinax lead to areas like Starport, 'Night Below' the Underdark Campaign expansion set, the Drow's Menzoberranzan, or the lost Dwarven city of Dylvwyllyn of 'Shards of the Day' adventure. My Alternate Darokin choice is the Remote town of Elstrich set B.C. East or NE of Elstrich and have it as a logging village that cutting timber to help rebuild Fort Anselbury(WOTI) or Fort Highlakes(after 1010 a.c.) I chose these sites due to the Malpheggi conection, I like the idea of having the creatures of the Malpheggi Swamp as the minions of the Beholders who raid villages around the swamp only on orders of their masters, the Beholders, which good for running a long Campaign.
Now for those of you who wish to use Glantri, I can only thnk of two Places that would work. One is Vyonnes in the Principality of Nouvelle Averoigne. Forest are few in Glantri(see Glantri:KOM) after the Meteor that crashed on Corran Keep. For Eye of Pain, rumors of a beholder near the Principality of Morlay will drive the White Wolf the offer a reward to remove this Beholder in order to protect his creatures of the night. Vellinox the Mage in the module, can blend in and out of the normal glantrian crowd, if your group was to try and track him there. You can have the Unblinking Eye be the allies or rivals to the Unseen Hand(Assassins' Guild) or the Fellowship of the Pouch(Thieves's Guild). For Eye to Eye, You can connect the Tunnels leading of the Ith K'hinax to the Underground Broken Lands or even to the Shadow elves tunnels, or 'Night Below' and you can based under the Gigantic plateau next to Glantri, and Sind The Results from the series, is bond to given your mage PC, minor title, and a possible seat on the Glantrian Parliment.(NOT THE COUNSEL OF PRINCES!!) My other choice is Igorov in the Principalty of Baldovia, run it as normal, because Igorov is possibly too far from the influences of the Unseen Hand, and the Fellowship of the Pouch.
I Have very little info on Thyatis(only the Mainland, in detail.) My choice is Greenheight in Machetos Estate(King Stefan's old estate), Run the module there as normal. For the Northern Reaches, I chose Backwater, and Snowvale. Backwater in the Soderfjord Jarldom, for the great marsh, by which the beholders has some minions to kick around, and Snowvale for the orcs and goblins living in the Makkres mountain range. My last Choice is Ranwood with B.C. north on the Landersfjord river in Vestland.
My idea for this region is the have the Inheritors of the Flame be either the Allies or Rivals to the Unblinking Eye,(your choice.) Here are my choice for the Savage Baronies. Cuidad Morale, Torreon with Villavieja playing as B.C. For Narvaez, Cuidad Quimera with Puente-Guardiana as B.C. Cuidad Real, Gargona with Bellavista as B.C.(Pronounce Bay-ya Vista a spanish adaption to Burke's Crossing rather than Juan's Crusando,) place the village of Bellavista next to Bosque de los Ojos(Translated from Spanish: Forest of the Eyes, If this doesn't clue your Pcs to Beholders living nearby, nothing will [no offense]) Cuidad Tejillas with Paso Dorado as B.C. And finally, if you wish for a wild west-type fantasy adventure, Here's Smokestone City with Little Big Rock as B.C.. The Gobliniod of the Yazi can pose as Beholder minions.