I've just completed my Priest of Lathander 29 -> Mage 30 build and wanted to share just how effective it is and how it clearly trumps any contenders.
Human, Priest of Lathander, Male, True Neutral
S 14, D 18, C 9, I 18, W 18, C 15
- Strength just enough for carrying, Priest spells sort it out soon enough
- Low con as 16 maxes out bonus HP anyway and DUHM does that
- 15 charisma gives 20 with friends spell to buy more scrolls sooner
Flail, Sling as weapons to start. By the end every weapon proficiency and style will be covered.
Part 1: beginning.
What makes this build so effective is that you can get enough XP without fighting to be able to summon skeletons who can clear the first areas easily when supported with hold person and hold undead.
Part 2: Aerial Servants
By the time you enter kresselacks tomb you get your first level 6 slot which let's you summon an aerial servant. Immune to normal weapons, multiple attacks per round, up to 40 damage per hit is amazingly good. The only place they struggle is against cold wights on Presio's level. Feel free to sanctuary past them and just kill Presio and his apprentice and come back at level 25 to destroy them with turn undead.
I never bothered to destroy the cold wights so only hit level 29 cleric on returning to Kuldahar but it seems likely if you did return to destroy them then you'll hit level 29 much sooner and have the option of summoning 6 aerial servants and then dualing and letting them destroy most of a level to accelerate you through the first few levels of Mage.
Part 3: Mage
Upon dualing buy whichever scrolls you like in Kuldahar and then jump over to Lonelywood and do as many quests around town as possible. With high charisma you can resolve the family quest and avoid being attacked when you visit the barbarian camp.
Burial Island is the scariest part of this walkthrough. Try to have invisibility, invisibility potions, shadow door, dimension door, protection from magic energy, spell deflection and mislead. Visit the Necromancers tower and buy at least one spell that you like of each level. Rest there and then haste or oil of speed and run invisible into the caves and look for Mebdinga at the bottom left. Grab the Tribal Symbol and immediately use an invisibility potion and run north to find the stairs up to Hjollder (if unsure scout out the area before going to Mebdinga - if you take contact other plane can pick up the nice suit of armour).
Talk to Hjollder and invis again and then dimension door to the mainland and run like hell to young ned and get off the island. This will leave a nasty surprise for our next visit to burial island but by then we'll handle it fine.
Back in lonelywood make sure that we've updated the bad wolf quest and then head to the gloomfrost.
At the gloomfrost expect a little trouble with the salamanders. Bottle necks, summons and cloudkills are your friend here. After them invis and pick up tiernons shield and proceed through the gloomfrost past everything to Tiernons. Have him upgrade the shield and make the fist of the Gloomfrost and Glacier Rose for you. Proceed to the hag invisible and with boots of the north equipped. We'll return later for the necklace.
Completing quests here and clearing out the quests in lonelywood (glacier rose, luskanites, bad wolf etc) and clearing the barbarian camp (protection from normal weapons and fireshields) pushes you to Mage level 30 and the completion of this guide.
After that you can return to Burial Isle and smash the undead here with any strategy that you like (priest buffs, Mage defenses, boon and IH let's you womp with 3 white doves or just stick with summons, skull traps and sunfires).
This guide is aimed at producing the most powerful character possible in the game - a level 29 Priest of Lathander/Mage 30. It is clearly more powerful than any single class and probably more powerful than any double or triple multiclass.
The build is superior to a FMC IMO because of the ease of leveling (no struggle street until animate dead, turn undead gains relevance sooner, Mage levels eventually overtake the triple class). Considering the build has a variety of ways to gain 10 APR there really is little advantage to the fighter class except for HP but the build has a healthy pool anyway which is quickly augmented with buffs. All saves end up well below 0 as well due to Boon (among other things).
A Gnome Illusionist/Cleric is a trickier comparison. It's easier to level than the FMC initially gaining animate dead sooner and has better saves and AC due to the helm. Eventually it'll gain cleric level 30 which is a slight advantage but will take significantly longer to get Mage level 30.
The build needs ~15m xp to complete compared to 20m for Cler/Ill multi and 30m for FMC multi.
Something else that has occurred to me is that the above walk through could be applied to a number of different dual class combinations with similar results:
Beastmaster -> Cleric
Priest of X -> Mage
Priest of X -> Ranger
Priest of X -> Fighter
Priest of X -> Thief
Priest of Talos 29 -> Mage holds some interest for me as I think evil clerics get a few spells that my neutral cleric missed (where's my Gate??). Beastmaster -> Cleric sounds like a lot of fun with Druid spells and summons galore though there is precious few clubs and staves to use.
I never would have thought that I'd complete a 29 -> 30 dual in IWDee but it's actually one of the most fun characters I've played and remains an interesting and varied play throughout. Well, until the dual is complete, now I have to see if I can be bothered to finish the game with my "perfect" build. I guess I can just invis rush the boring parts ha.