Each planeswalker has a signature spell that corresponds to that planeswalker:
Fblthp was a homunculus first seen in Totally Lost from the first block of Ravnica. Since his first appearance, he had gathered a large fan following and had been found hidden in the artwork of several cards. In War of the Spark, he received his own card, Fblthp, the Lost. This reveal came along with other native legendary Ravnicans getting their own cards (see War of the Spark#Cycles).