Ditto

A shapechanging mass of jelly which once resided in the Underdark where the lines between the Abyss and the Shadowfell crossed. Potentially a cross between a shapechanging fiend and a telepathic creature of the Shadowfell, "Ditto"'s true origins are unknown, as is the question of whether more of these creatures live deep in the caverns.

Encountered in the Shrine of the Spider, this creature was able to see into the memories of those who approached it and recognise the monsters most memorable to them and take on their forms. Though it was limited to a single form, "Ditto" was able to combine memories of multiple creatures into a single amalgamation in order to recreate the memory. This was seen when when "Ditto" became a "Nightwalker" from the memories of two adventurers that had the abilities carried by a "Boneclaw", which was another creature present in the memorable battle.

The creature's shape seemed to have no limitations, aside from the length of time that it would take to become the monster. Upon transforming, "Ditto" would entirely become the monster that it was mimicing, though it could never hide its true, fiendish nature from divine or arcane guidance. "Ditto" did seem to be limited by arbitrary concepts, such as legendary status. The creature was not of legendary status itself, but if it took on the form of a creature deemed as such, it would adopt that creature's abilities to seemingly shrug off failure. Similarly, its ability to act quickly wasn't limited by the abilities it had in its base form.

When encountered, "Ditto" took on four forms before it was finally defeated and returned to its true amorphus shape. Initially, it was a Marilith that was able to cast a darkness spell, an ability unknown to this fiend. Following this, "Ditto" retreated to its safe haven where it seemed to be changing its shape. It was then encountered as a "Nightwalker", which had the intelligence and combat prowess of a Boneclaw. Upon being slain, it became a "Molydeus", which didn't seem to have any particularly unique abilities as the monster was encountered alone. Finally, its final form before being slain was a recreation of the Waypoint adventurer Chains after he was possessed by the Shivering Maw, "Unfettered Chains". Upon being defeated, "Ditto" dissolved into a goo that may have been used to briefly copy its shapechanging abilities upon consumption.