Many games, especially free-to-play games, contain advertising. Instead of paying for the game with money, you pay for it with your attention. People are more willing to part with their attention than with their money, and often aren't aware of how much time they are giving to these in-game advertisements.
Advertisements by themselves are not necessarily a Dark Pattern, but they can become one if they are made too intrusive and interrupt normal gameplay too frequently. For example:
1) Ads that cannot be skipped.
2) Giving rewards for watching ads.
3) Tricking you into clicking on an ad by placing it in a place where you might accidentally click on it.
3) Disguising an ad to make it look like part of the game.
Games can sometimes treat watching an ad as a form of in-app currency to enable other dark patterns. For example, some games allow you to replenish your energy by watching an ad. This is a form of Pay to Skip
where you are paying with your attention instead of money.