I do not know where you can find information (I have forgotten the supplements, I fear.)
However, I can relate what I remember.
Larloch is a Netherese mage who survived the Fall, and by the power of undeath has remained on Toril to this day.
Larloch is a lich of exceedingly great power (around 30th to 40th level as a 2nd edition wizard); it is rumored that Mystra herself bequeaths to him ancient secrets (ala, access to 10th level spells and the like) in return for something (what, I do not know, but it's something obviously very important.)
Larloch has a literal army of liches (each 18th level or higher, as usual) under him; there are dozens of these liches enslaved to him.
Larloch has used some sort of magic to bind all these liches in slavery to him. They do his bidding, will destroy themselves at his command, and Larloch can see through their eyes, hear through their ears, and cast spells through them - using either their memorized spells or his own spells, at will.
If an individual lich is destroyed, it does not harm Larloch, and he cannot be attacked at all through these minions - at least, not by any spells I am aware of, nor by any standard psionics.
A Wish spell or similar powerful magic might break the compulsion upon a single lich, but what a freed lich would subsequently do, I am not certain.
The lich minions command a Necropolis, composed of literally thousands of powerful undead, and maybe hundreds of thousands of weak undead, which is buried under a mountain, and sealed away from intrusion by the most powerful magics Larloch could devise.
Apparently Larloch, like most liches, is extremely anti-social and hates intrusions upon his privacy, regardless of the nature of the intruder or the reason for the intrusion.
Ed Greenwood has said, to paraphrase him from web material put out by him, that Larloch was meant as Something To Be Left Alone.
Larloch is one of those beings who terrifies the likes of Elminster, the Simbul, and Tzass Sam.
Larloch, is capable of overwhelming any and all adventuring parties, regardless of their level or numbers - even an Epic Level party could not hope to face Larloch and survive.
I seriously doubt that even an Epic Level party of characters averaging 60th level, equivalent to the supposed Deities, could hope to take on Larloch and win.
Victory against Larloch would require some sort of epic campaign just to find the tools and allies needed to destroy them, and to persuade them to actually attempt the deed.
Artifacts, relics, the tarrasque, and the help of three or four of the phaerimm, or three or four of the sharn, plus help from the Chosen of Mystra, or three or four of the Zulkirs of Thay, or perhaps all of the leaders of Halruaa combined, or the combined strength of all the elven mages of Evermeet, might be sufficient.
It also might tear the continent of Faerun apart, the battle to destroy Larloch. It might kill half the people on the continent.
I know that there is no direct supplement on Larloch.
My regrets that I cannot give you the names of the supplements with the information above; it has been some time since I perused them.
Edena