lichess.org and put setting to hard?
Where can I find EXTREME Chess tactic puzzles

I believe if you have an actual computer, Informant offers free downloads of their tactics and endgame sections of their Informant game collection books. Some of those positions are very deep with multiple variations. Also, on Android, there's an app called Perfect Chess Trainer, PCT, it's only a few bucks, and there's a huge tactics trainer section. Many of those are very complex with playable variations.

You mean, like THIS for chess.?
http://www.wtharvey.com/
Very hard Puzzles that require me to calculate 6-15 moves
Any websites?