In most of the (admittedly limited) cases I've seen of people talking about new limbs appearing, the verb used was to sprout:
A new pair of arms sprouted from his torso.
However, the image this creates is a new limb sort of erupting from the other body part, as if they were a new or natural growth. If they were there all along but concealed, by another body part or by magic or similar, or you want a more magical appearance rather than a seemingly-biological appearance, I would use different words. It depends what image your are trying to evoke. However, I'm not sure the word forth is actually helping you here.
Seraphim wings sprang from her back.
This suggests that they appeared suddenly, all at once, as if they were concealed or came into existence all at once. It also suggests that they opened, as that's the easiest way of conveying the sense of motion that to spring evokes.
Seraphim wings appeared on her back.
This suggests they weren't there one moment, then they were the next, but with no sense of movement.
Seraphim wings unfurled from her back.
This suggests they were always there, but somehow concealed. They were folded up (furled, now rarely seen but its negation to unfurl is still used widely), and then they unfolded.
Seraphim wings unfolded from her back.
This suggests that they were always there, but concealed - though probably not well-concealed. It suggests something more mechanical than unfurled does.
Seraphim wings grew from her back.
This suggests a biological process, but one that sounds less violent than sprouted.
Seraphim wings erupted from her back.
This evokes a similar image to sprouted, but more violent, possibly (but not necessarily) involving pain and blood loss.
I'm sure there are other possibilities, but that gives a range of terms that give slightly different mental images.
By the way, purely as an aside, you may wish to be aware of the range of opinions around the word seraphim; a lot of people now consider that a plural, as the Hebrew word it is based on is a plural, and the singular is seraph. Thus, the attributive noun suggests that the wings are "those of multiple seraphim", which is fine - it suggests they are like those of all seraphim. It's just a little non-standard. Seraph wings is another alternative, also as an attributive noun, or you can use the genitive seraph's wings. Also, some people think we should take the singular as seraph and then give it a regular English plural, seraphs, rather than the Latin always-plural term seraphim, which was copied from the Hebrew. Personally, I like the elegance of the Hebrew-style plural.