Some time in the late 12th century DR (around 80 years before 1370 DR), Thendarion was a wizard who engaged in a spell-duel with another mage. According to tales, his rival was still alive by 1370 DR, and was in fact a Zulkir of Thay, presumed to be Szass Tam himself. Thendarion lost and was cursed to be trapped in the form of a boot forever.
In time, Thendarion was able to modify the spells he remembered to gain some freedom. However, sheer boredom had driven him insane.
It was unknown how and when he came to Elgath's Provisions (established in the late 1340s DR), but he took up residence there and always returned. Shoppers came to think the place was haunted. Volothamp Geddarm wrote of him in Volo's Guide to the Dalelands. He was still lurking amongst the merchandise in 1370 DR.
A side-effect of Thendarion's curse was that he could never forget the spells he'd had in his mind when afflicted, but nor could he learn new spells. However, he eventually learned modifications to his familiar spells. He could cast spells freely and at-will, but they often went wild due to the curse. For example, a simple attempt to conjure light could well turn a tree blue, set it aflame, or turn it into a rain of dying fish.
Two spells remained reliable and useful: shapechange and teleport. With shapechange, he could take the form of any nonliving object of about the same volume as his standard boot form, in a manner similar to a peltast, such as a lamp or knife, but he apparently preferred a lantern or a coil of rope.
The curse could not be lifted even after several expensive attempts. It was said by some that the curse warned the caster of attempts to tamper with it, and might even transfer to whoever did so. The rumored identity of the caster also made such efforts dangerous.
Bored witless, Thendarion attempted to get himself bought by adventurers visiting Elgath's Provisions, changing shape accordingly, so he could go on adventures with them and have fun. He often attempted to help, changing form to whatever was needed or casting spells to assist, but the attempts usually caused chaos, making him a nuisance.
When bored of his owners, or when disposed of him, retired or died, Thendarion teleported back to the store to wait for a new opportunity. If his purchasers returned him to the store, then Elgath the proprietor always gave a full refund, politely but reluctantly. Elgath sought out mages to undo the curse and rid himself of Thendarion, but each effort did nothing but cost him.