Kazgaroth (sometimes Kazgoroth) was an aspect of Malar, who was a powerful, intelligent, evil beast opposed to the balance and the influence of the Earthmother on the Moonshae Isles. It would periodically ravage the land until it was defeated, only to return several years later.
Kazgaroth's natural form was of a huge Tyrannosaurus rex, but it could shape-change to other forms, including those of people whom it might kill and devour before impersonating..
It served Bhaal, corrupting the eponymous moonwell, enabling his avatar to enter the Moonshaes.
Kazgaroth took several guises. The first few were a cow, which attracted a bit too much attention, and a woman named Meridith, who drew too much attention because Kazgaroth used its powers to make the body of the latter guise more attractive. His first victim was a man named Erian who was a guard for the prince Tristan Kendrick of the Moonshaes. Afterwards, he changed into a dryad in order to lure Trahern of Oakvale. His next victim was Thelgaar Ironhand, king of Oman's Isle, of whom he took the identity so he could command the army of northerners in the attack of the southern Moonshae Isles, mainly the city of Caer Corwell.
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Niles' own sourcebook, Moonshae (1987), describes Kazgaroth as an "aspect of Malar", which agrees with many of Kazgaroth's properties, including the ability to spread lycanthropy and its strong opposition to druidic balance, but almost all later works identify Kazgaroth as an avatar or minion of Bhaal. Given that Malar was subservient to Bhaal it is not impossible that both claims are true.
Hall of Heroes (1989) refers to Bhaal knowing the goals of "his avatar, Kazgaroth." However, the use of the word "avatar" would seems to mean "agent" here, as the rest of the text suggests that Kazgaroth is an independent being with its own goals (which Bhaal happens to know of).