At the end of the turn if there are 9 or more Undead Characters in your Support Zone which were in the Support Zone at the start of the turn, Virus Dispersal becomes complete.
When Virus Dispersal is completed it sacrifices all Undead Characters in your Support Zone, applies Poison 2 to all non-Undead Characters, and all non-Undead Characters on the Battlefield become Undead and are raised to your Support Zone when they die.
"Why are they just piling up like that?" *BOOM*
Useful for 3p SoA when you simply play Hehkeem (The Corrupted) after it resolves, for 2p SoA one should focus more on extra life like effect for chars on the battlefield.
Additional poison while hadly game-breaking, is nice to have. 2 cost with combination of Overwhelming Dead makes reaching 100% relatively early if needed, while this card allows for delay to make best use of its effect, but it comes at cost of sacrificing firepower, keeping your zombies in support for 2 turns and it may be interrupted by other player with right deck. All of that makes it very balanced and interesting, great idea.