Jack, what is this thread and what is festerblight?
Good question Robin. I don't know your name so I'm calling you Robin. You can call me Batman if it makes you feel comfortable, but I prefer Jack.
Festerblight is basically getting all your buffs (Trinkets are a big part of this, you'll want a Fabled Feather of Ji-Kun for sure; and strength proc trinkets are generally your best bet) up and applying your diseases via Unholy Blight. After then, you'll be using Festering Strike far more often to keep your diseases up for the entirety for the fight. This doesn't mean that you just ignore your Scourge Strike like it's your child asking for that candy bar, you will to start using Scourge Strike a bit more often with your Death Runes about ~30 Secs (hehe) left. You spend your Unholy runes via Timmy roids (Dark Transformation) and Scourge Strike.
Your diseases take a snapshot of damage bonuses on your bosses, so you'll need to refresh them when those situations arise.
This means that you can not take Plague Leech and do the spec properly, that would involve resetting your diseases and we can't have that.
So spirit is the best stat right and how much of it do I need?
No you silly goose, spirit isn't your best stat. Currently, there has been no maths to say that there is any alteration in the stat priority from normal Unholy. This being said, the BiS is completely the same other then the trinkets. Festerblight BiS trinkets are Primordius' Talisman of Rage and Fabled Feather of Ji-Kun.
What add-on's do I need?
DK Dots and your prefered rune display is all that is needed.
When do I not use Festerblight?
If there are AoE situations, go normal unholy.
If there are immunity phases (I.E Spirit Kings Heroic, Elegon), go normal unholy. Disregard this rule IF it's a short immunity such as Lei Shei.
What talents do I use?
Plague Leech isn't useful ever for Festerblight. Pass.
Unholy Blight is the go-to choice for Festerblight, it's what makes the spec work. Take this.
Roiling Blood isn't useful for single target, Festerblight. Pass.
Tier 2 doesn't have a dps bonus. I reccomend Purgatory though.
Tier 3 the only benefit to your dps here is Death's Advance.
For tier 4; if there is major damage boost in the fight, Death Siphon will be the best choice, if not go with Death Pact since you'll always have your ghoul up.
For tier 5; RE sims the highest; this is believe to be because of the extra free time Festerblight has.
Tier 6; if there's anything in the fight that can loss of control then Desecrated Ground is a bonus to your dps. Otherwise go with whatever the fight needs.
As for Glyphs, there are no required choices and are all subjective. I reccomend Unholy Frenzy and Icebound Fortitude though so you can break out of stuns easier.