When a new round starts, it looks as if you're fighting someone completely different, like Youmu, but SPENGBAB IT'S A TRAP. One of his special moves involve throwing the victim into one of two stage fatalities from Eternal Champions: Challenge From The Dark Side (they function like quick time events to make it easier), and they are signaled by a red flash picture of Speng bab. He can summon stuff like headcrabs, as well as employ his endless repertoire of PSI-powers, at seizure-inducing levels and plays horrible sound clips of agonized wailing and screaming at completely random intervals.
Put simply, if you once feared but then grew desensitized towards Giygas, this version of him will rekindle that fear, permanently.