Although I've been trying to reverse engineer/test Necrosis, I didn't want to make an "official" post until we had blue confirmation that everything tested thus far was accurate (as opposed to simply being a bugged or unintended side-effect).
Luckily, we just got it!
Here's what I've been able to determine so far:
Necrosis: Your Scourge Strike, Festering Strike, Plague Strike, Soul Reaper, and Pestilence multistrikes deal an additional (3% of Attack power) Shadow damage.
-Multistrikes with the slated abilities (Soul Reaper, Scourge Strike, Plague Strike, Festering Strike and Pestilence) trigger the Necrosis event to occur.
-Although triggered by mulstrikes from certain abilities, Necrosisis itself is SEPARATE from the multistrikes that cause it. Its damage is in no way tied to the damage caused by those Multistrikes.
-The shadow damage that Necrosis deals can Multistrike on its own, can Critically hit and also benefits from the Dreadblade Mastery. The base damage dealt by the event is dependent on Attack Power (currently 3% of AP).
-Attacks with multiple components (Pestilence, Scourge Strike and Soul Reaper) have a chance to multistrike independently with each portion. Any of these multistrikes can trigger Necrosis.
Hope this helps with any ability modelling.