Okay, I had a look at it and there is indeed work to be done before he’s arena-ready. Things you should address include but are not necessarily limited to:
- There’s no physical strength or speed mentioned anywhere.
- “his saliva makes whoever he bites incapable of dying for short periods of time (max 8 minutes)” What? Mind explaining why that’s one of his powers and what purpose it serves? It seems counterproductive in a fight.
- I advise that you make him resistant but not immune to physical attacks. Some characters don’t have anything else. Sure, they can just choose not to fight you, but you wouldn’t want to limit your potential opponents too much do you?
- If his flesh is mud, what are his bones made of? Does he have any organs at all? If not, can he even die? There are few things that can damage mud to the point where it isn’t mud anymore.
- Terror gaze is very questionable. It’s pretty much an auto-hit unless the target somehow knows not to look into his eyes. If you change "may blind the caster" to "will blind the caster" and put a time limit on the hallucination as to when it wears off, it might be acceptable. Though you'll have to ask LeeRoy.
- Terror sphere B isn’t overpowered per se, but it makes it unviable for a group of weaker characters to gang up on you, which in my opinion is one of the funnest parts of the arena.
- Terror claw can damage someone’s soul? You’ll have to define what effect that actually has. Also, I understand that you made the heart and lungs immune to it to prevent easy kills, but what about the brain? Or the spine for that matter.
- Iron maiden needs more explanation. Is it an actual iron maiden? Can he cast it on his enemies? If yes, does it just materialize around them or does it have to chase them?
- For summoning magic, please check out the summoning rules in the first post, so you know the restrictions. You’ll also have to give an accurate description of each summon’s abilities. The basilisk can’t petrify people through eye-contact, that’d be way overpowered.
- Conjure weapon: just add a line saying that he can’t conjure a weapon into someone’s body.
- Terror beast transformation: does it have a casting time, energy cost or time limit? Otherwise there’s no reason not to use it, and it nullifies his only major weakness.
- Mass manipulation needs a limit on how big he can get.
- Non consistent form needs an estimate on how fast those things can grow.
- You’ll need an estimate on the explosive power generated by things he detonates. You may also wanna state he can’t detonate people, though I’m sure you weren’t planning to.
- Possibly the most important part: you mention explicitly that he has other abilities that aren’t stated. That’s kind of a problem, as you could just pull whatever is needed to counter a certain situation out of your ass, which is referred to as Batmanning. It’s not that we don’t trust you not to be unreasonable, but we’d greatly prefer if you write down all his abilities and limit yourself to only those stated. Of course, slight deviations of existing abilities are fine.

Phew, I think that wall of text sums up most of the problems. It should also be said, he’s really damn powerful. Depending on the specifics I asked for, he may end up being above tier 7, which is currently off limits, meaning you’ll have to make a weaker version of him. I don’t know for sure though, cause as I said a lot of his abilities are still too vague to tell.