So I've been offered, by the Gm, a chance to have a Plasma Gun as -starting- equipment... as it would perfectly fit my character's background (Ryza Skitarii) but only if I take a "Witch Mark" as a malignancy.

But all I can find is
" Witch-Mark." T he character develops some minor physical deformity or easily concealable mutation. It is small, but perhaps enough to consign him to death if found out by a fanatical witch hunter."

There's no... examples! I have no idea what it should look like! Where it should be! How 'concealable' is concealable?' Can they be removed? And googling has proven fruitless, so... help?

Could be anything warpy. A raised melanomic birthmark on your thigh that kind of looks like a symbol of Khorne. Maybe there's a small vestigal human face growing on your back. Or just a hole filled with sharp teeth. Something that resembles a purple octopus tentacle growing on part of your body (and regrows). I wont delve into Slaanesh territory, but for Nurgle, an old wound that keeps getting infected and infested with maggots even though you keep cleaning it and have it covered. I would make it something disturbing like that.

Something that is disturbing to the average person, but would make a Witchunter fanatic want to purify you in flames for.

First up...deals with the DM are a lot like deals with the Devil; be very wary of signing this contract!

To address your question, though, it's what it says it is in the description; a small mutation or deformity, typically easily concealed. Traditionally, a witch-mark would be a large or oddly shaped birthmark, but other things like a club-foot, hunched-back, hare-lip or other "unsightly" deformities could also be taken as "signs of the beast".

In DH, however, you've a lot more scope for getting creative, as demonstrated by Utherix. It also depends on how much you want this aspect of your character to come into play. Is it something you want to largely ignore? If so, have a weird birthmark on your shoulder and be done with it. If it's something you want to have to deal with regularly, have an extra mouth on the back of your head, or something, that forces you to wear a hood or helmet at all times.

In DH, however, you've a lot more scope for getting creative, as demonstrated by Utherix. It also depends on how much you want this aspect of your character to come into play. Is it something you want to largely ignore? If so, have a weird birthmark on your shoulder and be done with it. If it's something you want to have to deal with regularly, have an extra mouth on the back of your head, or something, that forces you to wear a hood or helmet at all times.

It also depends on just how much you want to push into OBVIOUSLY CORRUPTED territory. A birthmark is something that anybody could grow, but there are people in the Imperium that could still interpret it as a sign of tainted genetics. Considering that we're talking about the Inquisition here, there's some wonderful room for interpretation and even interpersonal conflicts when you have some people believe it's nothing and others are convinced your character has been touched by Chaos. Especially if not even your character him/herself is 100% sure about this...

REPENT!

Better yet, have this witch-mark be in a shape that could (by squinting) be considered a shape of some kind. To you, it looks like Ryza's standard, and you're super-blessed by the Omnissiah, while others see possibly Chaotic markings! Get that detail out there!

This is how Wars of Faith are started, donchaknow.