Some questions and Possible Spoiler below!
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Keep Going!
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I went into the little building next to the temple (1 map East of Burgost), I talked to the priest and he started screaming that evil was comming, then Deamonknight named "Ranthis" showed up with 2 crazy powerfull skelitens and started throwing Fireballs around as if they were snowballs! Needless to say it killed my group...

The next time I prepared but Kagen and Shar-Teal both died perminant deaths (I dont relode if I loose a character and cant reserect them, unless its my main character oviously and Minsc!...I am almost out of NPC's )

I talked to the 1 surviing priest and he said to talk to Kelddath Ormlyr (the head priest in the big temple on that map), so I did; he told me that the DeamonKnight was just a messenger and that there was a bigger and badder deamon named Lestraubimat who was going to come through a portal with an army of Fire Salamanders and other deamons! Kelddath told me that the deamon was going to arrive to the East and that I needed to hurry...

Ok, I have played through BG1 more times then I can count over the years, but this is a new one to me. What mod dose it come from?

I am going to copy & past my WieDU install information in here to show what I am using.

"// Log of Currently Installed WeiDU Mods
// The top of the file is the 'oldest' mod
// ~TP2_File~ #language_number #component_number // Component Name
~SETUP-VAULT.TP2~ #0 #0 // Vault Item Pack v4.0 (BG: TotSC)
~SETUP LOSTITEMSV1.TP2~ #0 #0 // ???
~SETUP LOSTITEMSV1.TP2~ #0 #2 // ???
~SETUP LOSTITEMSV1.TP2~ #0 #3 // ???
~MIXMOD/MIXMOD.TP2~ #0 #0 // WARNING
~MIXMOD/MIXMOD.TP2~ #0 #14 // Increase Gem and Jewelry by 500%
~MIXMOD/MIXMOD.TP2~ #0 #22 // Increase Potion by 750%
~MIXMOD/MIXMOD.TP2~ #0 #28 // Increase Scroll by 500%
~MIXMOD/MIXMOD.TP2~ #0 #35 // Add 150%/6/6/6
~SETUP-VAULT.TP2~ #0 #0 // Vault Item Pack v4.0 (BG: TotSC)
~SETUP-BG1ADVENTUREDS.TP2~ #0 #0 // bg1adventureds
~SETUP-BG1ADVENTUREDS.TP2~ #0 #1 // Harder Enemies
~SETUP-BG1-EASE-V2.1.TP2~ #0 #0 // Higher (500 items) stacks for gems/missiles/potions/scrolls
~SETUP-BG1-EASE-V2.1.TP2~ #0 #1 // Bag of holding and improved Erdane store
~SETUP-BG1-EASE-V2.1.TP2~ #0 #2 // Higher stacks for normal missiles in stores
~SETUP-BG1-EASE-V2.1.TP2~ #0 #3 // Maximum of hit points per level
~SETUP-BG1-EASE-V2.1.TP2~ #0 #4 // Improved spell learning for bards and mages
~SETUP-BG1-EASE-V2.1.TP2~ #0 #6 // Happy dual-classing
~SETUP-DS.TP2~ #0 #1 // Level 40 Rule Pack
~SETUP-DS.TP2~ #0 #0 // Dark Side of the Swordcoast for Baldur's Gate (Tales of the Swordcoast) Version 1.02
~SETUP-TGC1E.TP2~ #0 #0 // The Grey Clan Episode One: In Candlelight, BG edition v1.00
~SETUP-BG1-EASE-V2.1.TP2~ #0 #5 // Happy mode (no annoyed or angry NPC)
// Recently Uninstalled: ~IMPROVEDBAMS/IMPROVEDBAMS.TP2~ #0 #0 // Improved Inventory BAMs
"


Anyone have any answeres?

It is new to me as well...
Maybe DarkSide or BG1 Adventures? I don't know any of them, so just guessing.
One thing is sure: not Grey Clan.

I also posted the above post on the Chosen of mystra forum, and I was told that the encounter comes from the Vault mod... I did not get any more info then that this morning.

I have serched everywhere I can think of and still I cnat find the rest of the encounter