Dark Crystal

The Dark Crystal is an extremely dangerous block added to the game. It is unavailable via the creative menu but can still be crafted and obtained via NEI (The crystal horn, which is only used in the Dark Crystal is still available in the creative menu). Unlike Taint, it is a true virus block that spreads by "consuming" adjacent blocks and replacing them with the nigh-indestructible Hardened Dark Crystal (its inert form).

Results
The inert form is indestructible to even the most powerful nuclear weapons, only falling to Schrabidium bombs which destroy everything, and acid (also destroys everything) which is creative only. Hardened Dark Crystal does not harm mobs. It will spread and take over everything until everything is useless Hardened Dark Crystal, even bedrock, leaving everyone to be pissed at the person that deployed it while they all starve to death.

Prevention
The best means of prevention is obviously to not let yourself or anyone else craft it. If someone is stupid or crazy enough to deploy it as a last-ditch effort to "see the world burn", launch everything you have at it, especially Schrabidium bombs, as Fölkvangr fields are really the only means of destroying it effectively (Though in testing I have theorized that regular nuclear detonations force it to become inert, nothing solid has come up).

There is also a block called Pulsating Crystal which will harden all virus blocks in an 8 block radius. This method only works if the spread is still small.

Turning off virus blocks in the config file will prevent it and other virus blocks from functioning at all.