It's hard to answer without knowing exactly what Taint does, but making some assumptions…
Start by considering Taint to be a base state: something that just is, a part of the world, part of the background assumptions that the Company rules indicate exceptions to.
Effectively, allow Taint to reduce the Territory stat (or Sovereignty, if Taint is a mind-affecting thing) of a Company with land that isn't protected by Daylight Crystals. (This would either be across-the-board for all unprotected Companies, or as special stat-reducing events for specific Companies, depending on how this Taint stuff works in your setting.)
Then, you can model the crystals as Assets (Enchridion p. 104). Treating Taint as the base state of the environment, Daylight Crystals would be an Asset that does something depending on what Taint is. Whatever Taint does, consider the Daylight Crystals Asset to enhance the opposite. If Taint makes people disloyal, the Asset gives +2d when you Train and Levy Troops. If Taint makes people unproductive, then the Asset allows you, once a year, to permanently reduce Treasure by 1 for a permanent increase of 1 to Territory (modelled on "Permanent Underclass", Enchiridion p. 108), which simulates the ability to expand safe holdings by investing in more Daylight Crystals.
Alternatively, if Taint is a thing that turns villages evil or something dramatic like that, don't model Taint as anything: just run the Company as usual, but make the PCs make a "Installing and Maintaining Crystals" Treasure+Sovereignty roll each month to protect their holdings, failure on which generates a new, small, enemy Company (or two) that will be a thorn in their sides. Think of it as spawning homegrown cults every cycle. Then they can have the fun of tracking them down personally or sending in the army.