That depends, what you mean by free games? Games that are free-to-play? Indie games that are given for free for a short time as promotions? AAA Games but Cracked versions? Games that are so simple, their developers decided to not charge for them?

My answer will depend on what you mean.

does anyone knows any darknet games and do they even excist?

• Cracked versions of AAA Games, there probably are, but you can find them in the clear-net too (I won't recommend them though).
• Games that are made specifically to be published on Darknet? Probably they don't exist.