You gain 5% more of the Multistrike stat from all sources.
Your Scourge Strike, Festering Strike, Plague Strike, Soul Reaper, and Pestilence multistrikes deal an additional (12% of Attack power) Shadow damage.
Necrosis is a death knight ability avaiable at level 90 for those with the unholy specialization. It gives a slight boost to the multistrike stat, while increasing the damage done by that stat for particular abilities.
Your autoattacks deal an additional X% Shadow damage.
Necrosis was originally a Death knight talent located in the 3rd tier of the Unholy Tree. It was especially useful for dual-wielding death knights because more of their damage was from auto-attacks than for other specs.