Challenge Thaxll'ssillyia - Baldur's Gate 2 Side Quest
This is a side quest that I made up in Baldur's Gate 2 that is essentially an excuse to give you an entire boss write up for Thaxll'ssillyia. As you may or may not know, Thaxll'ssillyia is a shadow dragon that can be found in the Temple Ruins map. You'll most likely find Thaxll'ssillyia for the first time while doing the Investigate the Deaths in Umar Hills side quest since you'll have to run right past Thaxll'ssillyia to reach the Shade Lord.
One thing to note is that so long as you have the Shadow Dragon Wardstone in your inventory Thaxll'ssillyia will not attack you. You'll find this item while exploring the Temple Ruins and you can keep it even after you finish the Investigate the Deaths in Umar Hills side quest. Having this item will make the fight against Thaxll'ssillyia significantly easier because you'll be able to get into position and even place traps before engaging.
If you'd like some basic advice on how to play different classes in this game and general advice on what buffs to use and things of that nature I recommend you check out my BG2 Beginner's Guide. The information I share in that guide will help you with difficult fights like this. Also, I would recommend you check out the Thaxll'ssillyia Wiki page for information specifically about this boss.
Note: While Thaxll'ssillyia is a dragon and all dragons in this game are difficult fights, Thaxll'ssillyia is probably the easiest or second easiest dragon that you'll be able to fight in the game. If you have a good strategy and some half way decent equipment you'll be able to take him out as early as Chapter 2.
Before I talk about strategies to use against Thaxll'ssillyia let me first tell you what we're up against. Thaxll'ssillyia is a Cleric class that has 169 HP along with an innate 25% Physical Damage reduction and 60% Magic Resist. He also has a -13 AC with mediocre Saving Throws.
So, as for the fight against Thaxll'ssillyia the first good strategy I can give you is traps! If you have a high level Thief in your party using Time Traps, Set Trap and Set Spike Trap will allow you to deal a high amount of damage to Thaxll'ssillyia the very moment the fight starts. He won't set off the traps until you engage him so you can put them right ontop of him.
As for the fight itself, you'll want to buff up your characters as much as possible with all the best buffs you have. Improved Haste, Stoneskin, Iron Skins, Protection from Evil 10' Radius, Bless and even summoning pets like Animate Dead or Conjure Animals will help out a lot. You definitely don't want to forget Resist Fear or Remove Fear and Negative Plane Protection as well because these spells will counter a lot of the status effects from the dragon.
When the fight actually begins what I typically do is debuff Thaxll'ssillyia as much as possible with spells like Greater Malison, Lower Resistance and Breach. One Greater Malison one Breach and 2 or 3 Lower Resistance is what I typically cast on him. After 2 Lower Resist I will use Magic Missile on him and check how many actually hit him, if most of the Magic Missiles are resisted I will typically throw another Lower Resistance.
Once Thaxll'ssillyia is debuffed I typically start laying into him with spells like Magic Missile, Finger of Death, Flame Arrow and whatever else you can throw at him. Having spells like Spell Sequencer and Spell Trigger can also make this fight much easier since you can cast multiple debuffs at once or multiple damage spells at once. Combine that with other spells like Contingency and you can create a scenario that immediately at the start of the fight Thaxll'ssillyia will be loaded up with debuffs and hit for a lot of damage.
I could go on and on about different strategies and ways to kill this boss but I don't want to drag on (get it? drag on dragon...). For different strategies than I have provided on this page I recommend you check out the Thaxll'ssillyia wiki. They got some cool strategies like turning into a Mustard Jelly to beat him that I have never tried.
Upon his defeat Thaxll'ssillyia will drop Crom Faeyr Scroll and Shadow Dragon Scales. Both of these items can be used at the Cromwell Blacksmith in Docks District to craft powerful gear. If you'd like more information about Cromwell follow the link provided.
Aside from that, I have organized information about Thaxll'ssillyia into lists below. I figure it's easier this way to get information you're after.
Shadow Dragon Breath: This is Thaxll'ssillyia's worst attack, it will Blind you for 1 turn guaranteed along with applying a Level Drain that drains 4 levels. If you fail a Saving Throw against the breath then you'll get an additional 2 levels drained.
Wing Buffet: As with all dragons Thaxll'ssillyia gets a Wing Buffet attack that will knock back any target it hits along with dealing a moderate amount of crushing damage.
Dragon Fear: This is another universal dragon ability, Thaxll'ssillyia will use AoE fear occasionally throughout the fight. You should use Remove Fear at the start of this fight to prevent your characters from getting hit by this AoE fear.
Stoneskin: Thaxll'ssillyia can instantly cast this spell and will a few times throughout the fight. Using a spell to strip Stoneskin off will help a lot.
Haste: You probably already know what this spell is capable of since it's one of the most common and important buffs. Well, Thaxll'ssillyia gets it too.
Chaos: This is probably one of the more annoying debuffs that Thaxll'ssillyia can use.
Debuffs to use on Thaxll'ssillyia:
Greater Malison: I recommend you throw this at Thaxll'ssillyia at the start of the battle as it'll decrease his Saving Throws and make him easier to hit
Doom: This spell stacks with Greater Malison and will decrease THAC0 and Saving Throws
Lower Resistance: Using this on Thaxll'ssillyia will help a lot of your magical spells to land against him. Lower Resistance stacks too which is worth noting, you can use multiple against him to lower his resistance significantly.
Breach: This is the lowest level spell protection removal that can work against Thaxll'ssillyia and it won't remove everything. However, it's better than nothing.
Ruby Ray of Reversal: One of the higher level spells you can cast that will strip Thaxll'ssillyia of many magical defenses.
Pierce Magic: Much like Ruby Ray you'll want to cast this on Thaxll'ssillyia to strip magical protections from him.