POWERS

[untiltheend]

So. Canonically, Sebastian is a ridiculously overpowered Gary Stu godmod. Observe:

Enhanced senses, strength, speed, etc: He's far stronger (able to drag a horse cart full of people along at an inhuman running speed, can crush human skulls using only his hands), faster (once popped over to Finland from England in about fifteen minutes just to deliver a quick message) and more agile (can run up walls vertically, able to jump the length of an entire passenger ship) than a human, with far superior senses (can sense the presence of life - or lack thereof - through walls, can spot one single person in a crowd of hundreds). A normal, non-superpowered human, no matter how skilled, basically doesn't stand a chance against him if he decides to be serious.
Suggested nerfing: If it's okay, I'd suggest bringing him down to "still above human capabilities, although way more limited than in canon", simply because he'd be so angry over being reduced to a weak, pathetic human, and thus way harder for a Warden to work with. If that's not okay, then within human capabilities it is!

Obnoxious "Anything You Can Do I Can Do Better" skills. Pretty self-explanatory. He has to learn and practise new things like anyone else, but he picks them up fast and, usually, with ease... and on the off chance he doesn't, he's such a giant perfectionist he'll practise relentlessly until he gets it just right anyway. Being as old as he is he's something of a well of knowledge (although, perhaps tellingly, he's sometimes a little lost when it comes to how human health works) and pretty much the humanoid version of a Swiss army knife of skills. If it exists in the Victorian era, Sebastian can most likely do it.
Suggested nerfing: Intact.

Ability to create matter: He canonically re-creates an entire mansion out of thin air, in an instant, using no materials whatsoever. He also repairs smaller objects in the same way, and can create an entire banquet in the blink of an eye. His pretending to be human means he doesn't use this power much, however - at least not obviously so - and he's fully capable of doing all his gourmet cooking, top-star baking etcetera etcetera the normal, mundane way.
Suggested nerfing: Poof gone. Honestly, he uses this ability so sparingly as is, it's hardly even relevant.

Extremely fast healing and durability: He heals within seconds of being shot in the head or stabbed clean through the chest (it's still seriously painful for him even though not fatal), and you need special weapons to do any serious damage to him.
Poisons that are deadly to humans do nothing, or only briefly inconvenience him. He has no need to breathe or to have a beating heart (although he does both of those, because there are some limits to what people will let pass as "human" even in Kuroshitsuji.) He doesn't need to sleep, and seems completely unconcerned by the prospect of potentially going a human lifetime without eating.
Suggested nerfing: No need to eat or sleep. Healing somewhat slowed down? Immunity to poisons intact, aside from Flood-induced shenanigans, or characters with weapons or abilities that work on demons/monsters/everything ever.

Soul sensing: Being a creature that feeds on souls, Sebastian can pick out the presence of human souls within certain distances. So far the manga is vague on exactly how it works, but it seems this ability isn't precise enough for him to easily notice oddities in one soul in a crowd. Ciel's soul is different: they're bonded together by their contract, meaning Sebastian can always tell where he is.
Suggested nerfing: Intact (again, he seldom uses it, and I'll check with the mun in question before using it).

True form: Sebastian is, essentially, a shapeshifter. His appearance as a handsome butler is one he's donned because it's what works best for his current contract, and he could take on any appearance necessary. His true form can be summed up as "Eldritch Horror". His butler form is very much flesh and blood, however, and it's not a form he'll abandon as long as he's still contracted to Ciel. In canon, he'll occasionally show glimpses of his true form, usually as a scare tactic, but aside from that he never shapeshifts into anything else.
Suggested nerfing: Confined to his butler form.

Assorted random powers because everything mentioned above isn't enough: He's shown manipulating flames without burning himself, making flowers bloom unnaturally fast (...okay) and can apparently put people to sleep at will. Again, these powers seldom get used, since Sebastian a) typically doesn't have to resort to using them, and b) tends to be more of a physical fighter anyway.
Suggested nerfing: Poof gone.