Kaiju-Lich
Imagine a kaiju, like something out of Godzilla, but it has both the physiology and abilities of the Lich (like the one from Adventure time). The more souls harvested into a runic pot that is then activated in a dark ritual, the stronger the aforementioned Kaiju-Lich is. Even I don't fully realise just how terrifying the implications behind this concept are.
This titanic abomination has all the standard kaiju capabilities, such as causing city/country/higher-wide destruction with its sheer size alone, followed by whatever monstrous abilities it may possess – stacked on top of necromancy, voodoo and other dark magic. And let's not forget how it can literally induce death within any mortal lifeform with just a glance.
Think about it: this thing can reanimate the corpses of dead kaiju, and the more souls the Kaiju-Lich consists of, means it is able to conjure greater amounts of kaijus in a single act, as well as manifest a larger form than what it would do if it manifested with a lower amount of harvested souls.
If that isn't terrifying, I don't know what is.