Massive Spellcasting Out of Sight
There is actually a way to make this battle really easy.

Just move your entire party up to the plate where the portals are.

Then, move a fast character down and start the battle and then run up the platform luring the demons he summons. When they are dead, move your spellcasters down the stairs but make sure Demogorgon does not see you. Then you cast all the cloadkill/death fog/abi'dhalzims horrid wilting spells you have and sit back and watch him die. It takes quite a few to kill him so make sure to have lots of 'em prepared before you start the battle.

Submitted By: NoctoZ

Trap 'Em
Defeating Demogorgon was a cinch. When you first talk to him, tell him you will use the scroll and then do so. He will then tell you to go and check the portals. He will remain invisible and neutral until you talk to him again. This will give you an opportunity to prepare for the upcoming fight. The fastest and easiest way to beat him is to have a thief lay traps in the center of the room where he appears. Set as many spike and normal traps as you can. Be careful not to use the timestop trap, as he is immune to the effects. Set as many traps as the game allows, and then summon some pets just in case. When all is ready, speak to him again. Make him appear as the demon and he will turn hostile. The traps will all spring at once as soon as the dialogue box closes. In my case the traps killed him instantly. If they do not, he will be very close to death and just have your fighters and pets take him down. With this strategy Demogorgon proved to be no challenge whatsoever. This strategy also works with all other enemies that start off neutral.

Submitted By: Sunter21

Spell Triggers & Timing
First I would like to say that my group has 4 wizards and is magic heavy but I believe this same strategy could work with only one spellcaster (should be at least 23 or so). For spells, you will need Spell Trigger, Lower Magic Resistance, and Improved Alacrity (Chain Contigency, Breach, and Pierce Magic would probably be a big help, especially if you only have one mage). If you have two mages, have both of them setup up a Spell Trigger with these spells: 2 Pierce Magic (Lowers Magic reistance and dispells spell protections) and 1 Lower Magic Resistance on one mage. On the second mage, have 2 Pierce Magics (to be on the safe side) and 1 Breach. If no second mage, use Chain Contigency to replace the second mages spell trigger and set it to go when an enemy is spotted.

Now, when you enter Demogorgon's Lair at fight time, have the mages walk down one set of stairs and your fighter/tanks down the other. Make sure both groups stop out of sight of Demogorgon but as close to the bottom as possible. Also make sure all your buffs are cast before entering Demogorgon's Lair. Before entering battle, make sure that you have BG2: ToB setup to auto-pause whenever a spell is cast. Then have mage(s) cast Improved Alacrity and then have both groups go to bottom of stairs simultaneously. Pause as soon as Demogorgon is in sight (or better yet, have BG autopause when an enemy is sighted). Have fighters/tanks rush at Demogorgon (Weapons of +5 enchantment or recommended though may not be required. Haven't tested with a lower enchantment). At the same time, have mage(s) use Spell Trigger(s) on Demogorgon (Chain Contingency will automatically cast when Demogorgon comes in sight). Spell Triggers/Chain Contingencies will have net affect of lowering Demogorgon's Magic Resistance by approximately 110% (assuming a mage level of 20), dispelling all combat protections (such as Stoneskin) and dispelling 4 8th level or lower Spell protections.

If you do all this fast enough, it is possible to get off Spell Triggers before Demogorgon actually uses any spell protections so you might want to wait for him to cast protection spells (which will happen quickly) before using your own Spell Triggers. Timing will be critical. After all this is done, not only will your tanks be able to start beating on Demogorgon (Warrior-High Level Abiliy use is recommended) but he will also by vulnerable to pretty much any spell you want to cast at him. I then used Spell Strike on him to be on the safe side but with good timing this shouldn't be neccessary. After all this, start casting the most damaging spells you have at him(especially ones that don't allow Saving Throws). I recomend lots and lots of Abi-Dalzim's Horrid Wilting next to him since there is no chance of harming party members and spell protections don't affect it. With my own magic heavy group I was able to cast somewhere in the neighborhood of 8 Abi-Dalzim's but use scrolls if your group isn't quite so magic concentrated. With Improved Alacrity on all spell casters you will be able to cast dozens of spells while Demogorgon only makes a couple of attacks. Vecna's Robes and Amulet of Power are highly recommended since they decrease casting time. A wizard wearing both can cast any spell with a castime time of 5 or less instantaneously and be able to cast up to 12 spells in a single round (maybe more).

With all the damage your tanks and spellcasters will be doing, Demogorgon should go down in a matter of rounds (I think I killed him before my Improved Alacrity spells even ran out). NOTE: Do NOT cast Time Stop spells. Demogorgon is immune and you will be affectively making the fight one-on-one with Demogorgon. I used this spell the first time I tried the fight and watched Demogorgon kill my entire group besides the caster before the spell ended. If Demogorgon summons up a couple of Marileths before you manage to kill him, just have a spell caster cast a Lower Resistance at each one before continuing with Area-effect damaging spells. The spells that kill off Demogorgon will kill them just as easily once their Magic Resistance is lowered.

Submitted By: Menion Leah

The Power of Simulacrum & Summmoning
The way I took care of Demogorgon was pretty simple, and the battle itself took less then 2 minutes. First thing I did was talk to him, and basicly say I would bring down the knights, so I had a way to walk around his area without being attacked. Then I placed my 3 warriors toward far left of screen and had my thief place all his snares and traps right in the middle of the room where he forms. Even though you have to have +4 weapons to hurt him, the snares do very well. I then used the Simulacrum trick. I made sure my mage had the wand of summoning and his copy then used all 4 charges of the wand and swamped the middle of the room. I made sure my 3 fighters were out to the side, and had my mage in far right side of room. I sent my other 2 people up the stairs since they would just be in the way. I started my mage to cast dragon breath at the middle of the room and then instantly spoke to the middle of the room and demanded Demogorgon show himself. When he did, the traps brought him down to badly hurt, then the dragon breath hit and took him to nearly dead.

All the monsters then attacked Demogorgon and so did the 2 demons he conjured. Then I cast ruby ray of reversal onto Demogorgon with my mage, and the 3 fighters finished off defenseless Demogorgon while he fought the monster swarm I caused. He was dead quickly and it was easy to finish off his lackeys. All in all it took perhaps 2 minutes and none of my characters took more then a few hits from his summoned demons. The trap trick works great against any enemy in which you know where they will appear, or walk over. Works great against dragons, liches and other nasties.

Submitted By: Rick DiNovo

Don't Stop Time
First you need the best weapons you can find,(my favorite is Foebane +5) and make sure you have are at least lvl 25. Now, Time Stop will kill you. If you cast that like always only you can move except this time you *and* Demogorgon can move. So then your mage dies... I use the best weapons with haste, improved alacrity, energy blades, lower resistance, protection from normal and magical weapons, project image, and some other defense spells, IE: protection from magical energy.

Demogorgon is almost always summoning creatures, you will not have fun being killed by a glabrezu when all of your power is being focused on a giant two headed baboon. Use the raised platform and stairs to your advantage. Have your fighters at the base killing summoned, and have your Casters and Archers doin what they do best. I have also fouond if all of your guys go to the top of the stairs by the transports, you can pause and un pause and try resting while it is paused. you can do this in the city also. if you rest and a guard wakes you up and you have had it paused when you rested, you can keep resting until you get your 8 hours in!

Submitted By: Drizzt Do'Urden

Seal Him Up
I must be the only person who did not kill Demogorgon. My PC (19th lvl paladin) remembered being told by the Vigils not to destroy the Imprisoned One so when he stepped into the final level of Watcher's Keep, he spoke with Demogorgon to try and gather information as to why. While he sensed evil in nearly every pore of his body, he did not sense deception when Demogorgon revealed that the Vigils were not quite truthful in their claims and that my PC was being played for a patsy. When he returned to the Vigils and told the them that the Imprisoned One had been destroyed (when, in fact, he hadn't been), they panicked and went to investigate. Lying to them wasn't a very honorable thing to do but my PC held on to the theory that if his word not to destroy the Imprisoned One was good enough for the Vigils, then his word to Demogorgon to lie to the Vigils was equally as good.

Once the Vigils left the top of the keep, a ghostly messenger (for lack of a better word) of Helm appeared and apologized for my being led astray by the Vigils. She then told the story of what the Vigils' charge was supposed to be and how these particular ones failed in the eyes of Helm. She gave me 3 options: walk away, go down & assist them, or read the real scroll (which would seal the keep and maintain Demogorgon's imprisonment). After seeking counsel from the rest of his party, the answer became crystal clear. While my PC was wronged into breaking all the seals to the keep, he was obligated to right the wrong for the sake of all Faerun. He read the real scroll which resealed the keep and prayed, albeit briefly, for those inside. The messenger again apologized on behalf of Helm and vanished. I don't recall how much experience each party member received, but it did level up a few of them. The party's reputation did not suffer nor did any alignments.

Submitted By: Elrin Britestar