I'm trying to make taint spread to a certain area in my world, as in, to a small NPC structure. Is there any way I can do this? I even tried using MCEdit to set the biome, but there's no "enter biome ID" thing, and Tainted Land isn't listed.
From How Taint Spreads on Bayrattack's link:
A local infection will not penetrate the ground and only infects the blocks on the surface ... The infection also only spreads to the blocks next to other already infected blocks ... If there are blocks the Taint cannot infect adjacent to infected blocks, the Taint will not be able to spread beyond them. As will be shown later, this can be used to create barriers that can temporarily stop a local infection from spreading. Taint will mostly infect naturally occurring blocks ... Taint will not infect most player-made materials
From this, you can sculpt a "path" of naturally occurring blocks, i.e. dirt, surrounded by man-made blocks, i.e. wooden planks, to the intended area.
Depending on your version of Thaumcraft, you can also research Concentrated Taint and drop it in the structure you want tainted.