Cards like Black Lotus, the Moxen, Ancestral Recall, and Time Walk remain valuable because they are both iconic and extremely scarce
The design for this was based on Mistform Ultimuss ability, and it was later featured on creatures from the Lorwyn block like Shapesharer
On the other hand, the specificity of the effect may keep the price high even if it doesnt catch on
The first two chapters of this saga are good enough at 3 mana, creating a creature with the possibility of ramping you and letting you pitch two cards for some draw
Your creature might not even survive
Legendary creatures have a whole set of rules that apply to them, but for the purposes of this article, all you need to know is that a legendary creature has the Legendary Creature text on it, like [c]Kenrith, the Returned King[/c] below