1) Erebos doesn't impact the board "at all" and its ability is "weak".
2) Ob Nixilis is almost legacy playable. And I like him a lot in my Walker Fit deck. But unfortunately, he is really slow and 5 CMC is pushing really hard.
At least his -3 impacts the board.
You might also want to look at/test Vraska. Her +1 is far worse but she can kill any non land permanent (Jace) and she can use her ultimate a turn sooner. But to my taste her ulty is worse than Ob Nixilis.
3) If you really want to go "crazy", the PW I would also try is
Sorin Markov.