The Forgotten Sons
I like the name, very Unforgiven. The IA could do with some grammar mistakes like the "its" as possessive so "the Curse of the 21st Founding showed its (not it's face)"
Perhaps a small little fact file about the Chapter, tell which Unforgiven chapter they are from
Dark Angels
Angels of Absolution
Angels of Absolution
Angels of Vengeance
Disciples of Caliban
Guardians of the Covenant
Consecrators
Explain why they are called the Angels of Healing (Many Apothecaries?)
Chose from the list as to the parent Chapter, do a little research on the chapter, see which one suits you, which one has similar colour scheme (TBH I think that the colour scheme is a bit bland)
Explain more about the curse, even if it just rumors of howls at night, some indication of the Curse. The Blood Chains ritual sound very VERY Khorne like, it looks like either a) The Inquisition come along and declare them Traitoris or they turn to Khorne. I would suggest finding a little bit less extreme ritual
The idea of Puritywing is interesting, but I think that it should be that a small core of Pure Brothers taken from the companies to overseer the purity of the Chapter (a bit like Brother Corbulo). They should hunt down any information about how they could reverse the curse.
Golgotha, I like the name, but the idea of Sisters next door could involve conflict because of the curse. The Sisters could have some intrest about the Chapter after hearing of the rituals.
The Big Battle
A coalition of Chaos Marines attack them system (Reasons why? Maybe they heard of the curse and were trying to convert them over to the Dark Side) I think that maybe you should have only one renegade band attack them, as large as the coalition so you can have a nemesis for the Chapter. Fallen Angels in the force attacking the planet seems good, keep that one.
The name also is bugging me "Black Fingers". Stick with the "Black Fists", seems better.
As for the battles, maybe there had been smaller battles on other planets, getting bigger and bigger and then onto Golgotha. Explain the virus, where did they get it from? Ancient world? Research Planet? Explain the mutations that took place, did it create new ones or build upon existing ones. Maybe have stories of how this turned the battle (to either side). What about the Sister in the system. Did they fight against the Black Fingers/Fists or did they refuse to fight with the Chapter as a result of the mutations.
Overall a good idea (Writing a 21st Founding Chapter is very difficult) with some rough edges that can be smoothed out