A post on the Elder Scrolls Online website from last week explains that they've seen a spike in the use of "fraudulently obtained" digital game keys for The Elder Scrolls Online: Tamriel Unlimited, and are deactivating all game accounts created with "such stolen keys" beginning today (thanks HARDOCP). They have a very lengthy list of places to obtain legitimate keys, and say that they are contacting those whose accounts are being closed with instructions on how to go straight, albeit with an additional purchase. They also explain the issue with the keys being revoked:

The companies selling stolen keys make a practice of using stolen credit cards, or misappropriating credit card information from their own customers to buy codes from legitimate retailers. Often these companies have the lowest price available anywhere for the digital item they are selling. Customers who purchase from these vendors are at increased risk for identity theft.

Extreme discounting below the published prices on our website may be a clue that the digital key purchased for the game may have been fraudulently obtained.