Void. It's got an entire planet at its knees, reducing it to a shadow of its former self and, as a power of nothingness, it seems to be erasing things as it goes. Worse, it can get to Earth once it's finished with Cray. In Episode 103, its made clear that it's poisoning Aichi, who notices that his strength and even his drive to win the fight against Leon is draining. By the time the toll becomes great enough that he collapses, he can't even move. It's a power that saps others of everything and leaves them hollow. Seeing as this came from space, how many other planets could it have drained before touching down on Cray?
What's even worse is, it did that while in Fog of Doom form. Now LinkJoker have appeared, its physical avatars who seem to exist solely to conquer in Void's name. Once they do conquer you, they separate the strong from the weak, killing the latter and assimilating the former into more Link Joker forces. Their lore explicitly notes they've already conquered half of Star Gate, with emphasis on the fact that it was their target due to having areas which could be used to contain large numbers of civilians. In other words, take The Borg, make them the servants of an As Long as There Is EvilStraw NihilistEldritch Abomination and you'll have something like Link Joker.
It gets worse. By the late stages of the game, Leon loses the use of his right arm, which becomes black from how far Void's corruption has gone. If he wasn't stopped when he was, things could have ended so much nastier.
One very chilling thing in Link Joker is Kai's descent. Mind Rape and his fears aside, his fall reaches its horrifying climax when he stabs a copy he deems his former self. And the whole time, he is just as horrified, like he's not in control of himself.
Reverse Koutei. The other Reverse fighters were either mindless drones or just had their desires warped and amplified. But Koutei has basically been turned into an exact opposite of his regular self, with the shy, kind Nice Guy becoming a cruel, ruthless tyrant. Vanguard's villains have never avoided the Evil Laugh or the Slasher Smile, but seeing them on Koutei is all the more terrible because they're just so opposite to his normal self. This is without mentioning his ace card, Reverse Daiyusha. A Fallen Hero version of Great Daiyusha which has been corrupted by Link Joker, and imprisons its own allies to drain the opponent's power.
This image.◊ You'll never look at a loli the same way ever again.