As a shapeshifter, Sarth could appear as any living thing, but usually appeared as male. When not disguised as human, his preferred shape was that of a tso (a spidery monster with the head of an eel, native to the Outlands). He also took the form of a crocodile on occasion. While wandering the Sword Coast, he disguised himself as a poor and confused priest of Talona.
Sarth had magic items and other loot stashed in various hiding places throughout Faerûn.
Sarth repeatedly traveled from the Plane of Shadow to Faerûn, took up apprenticeship with a wizard willing to teach, learned all he could, and then killed his tutor and stole his or her magic items, books, portable equipment and pocket-able wealth. After hiding his loot in various caches across the land, he retreated back home until he felt safe for another foray.
He roamed about the Sword Coast region disguised as a bewildered priest of Talona, getting to know the area so he could escape undetected if the need arose. He also used his itinerant travels to learn the location of great wizards whom he intended to subvert, plunder, and consume.
Occasionally a Zhentarim mage or Red Wizard would discover one of his hiding places and take his stolen magic for their own. When this occurred, he methodically hunted the perpetrators down and killed them.